Clark gained an early appointment to the USMA in June 1913 at the age of 17, but lost time from frequent illnesses.^[8] Known as "Contraband" by his classmates, because of his ability to smuggle sweets into the barracks,^[1] while at West Point, he met and befriended Dwight D. Eisenhower, who lived in the same barracks division and was his company cadet sergeant. Although Eisenhower was two years senior to him, having graduated as part of the West Point class of 1915, the two formed a friendship. Clark graduated from West Point on April 20, 1917, exactly two weeks after the American entry into World War I, and six weeks before schedule, with a class ranking of 110 in a class of 139, and was commissioned as a second lieutenant.^[8] He graduated alongside young men such as Matthew Ridgway, J. Lawton Collins, (both of whom later became U.S. Army Chief of Staff) Ernest N. Harmon, William W. Eagles, Norman Cota, Laurence B. Keiser, Frederick A. Irving, William C. McMahon, Bryant Moore and William K. Harrison.^[9]

Like his father, he decided to join the Infantry Branch. He was assigned to the 11th Infantry Regiment, which later became part of the 5th Division, where he became a company commander in Company 'K' of the 3rd Battalion, 11th Infantry, with First Lieutenant John W. O'Daniel serving as a platoon commander in his company.^[9] In the rapid expansion of the U.S. Army during World War I, he rose quickly in rank, promoted to first lieutenant on May 15 and captain on August 5, 1917.^[10]

In late April 1918, shortly before Clark's 22nd birthday and over a year since his graduation from West Point, he arrived on the Western Front, to join the American Expeditionary Force (AEF).^[11] Arriving with his company at the French port of Brest on 1 May, his 22nd birthday, the next few weeks were spent in training in trench warfare under the tutelage of the French Army and soon afterwards the division was inspected by General John J. Pershing, the Commander-in-Chief (C-in-C) of the AEF on the Western Front.^[11] Serving in the Vosges mountains, the Commanding Officer (CO) of the regiment's 3rd Battalion, Major R. E. Kingman, fell ill and Clark was promoted to acting battalion commander on June 12, 1918, with O'Daniel taking over command of Clark's company.^[11] Two days later, when Clark's division was relieving a French division in the trenches, he was wounded by German artillery in the right shoulder and upper back, knocking him unconscious; the soldier standing next to him, Private Joseph Kanieski, was killed. They were two of the first casualties suffered by the 5th Division during the war.^[12]

Despite his injuries, however, Captain Clark managed to recover within six weeks, although he was graded unfit to return to the infantry.^[12] As a result of his convalescence, he was transferred to the Supply Section of the First Army. In this position he served with Colonel John L. DeWitt, and supervised the daily provision of food for the men of the First Army, which earned Clark recognition at the higher levels of command.^[13] He stayed in this post until the end of hostilities on November 11, 1918. He then served with the Third Army in its occupation duties in Germany and returned to the United States in June 1919, just over a year after being sent overseas.^[13]

The Italian campaign of World War II consisted of Allied operations in and around Italy, from 1943 to 1945. The Joint Allied Forces Headquarters (AFHQ) was operationally responsible for all Allied land forces in the Mediterranean theatre and it planned and led the invasion of Sicily in July 1943, followed in September by the invasion of the Italian mainland and the campaign in Italy until the surrender of the German Armed Forces in Italy in May 1945.

It is estimated that between September 1943 and April 1945, 60,000–70,000 Allied and 38,805–150,660 German soldiers died in Italy.^{[21][nb 1]} The number of Allied casualties was about 320,000 and the German figure (excluding those involved in the final surrender) was over 330,000.^{[21][nb 2]} Fascist Italy, prior to its collapse, suffered about 200,000 casualties, mostly POWs taken in the Allied invasion of Sicily, including more than 40,000 killed or missing.^[23] Over 150,000 Italian civilians died, as did 35,828 anti-Fascist partisans and some 35,000 troops of the Italian Social Republic.^[24][25][26]

In the West, no other campaign cost more than Italy in terms of lives lost and wounds suffered by infantry forces of both sides, during bitter small-scale fighting around strongpoints at the Winter Line, the Anzio beachhead and the Gothic Line.^[27] The campaign ended when Army Group C surrendered unconditionally to the Allies on May 2, 1945, one week before the formal German Instrument of Surrender. The independent states of San Marino and the Vatican, both surrounded by Italian territory, also suffered damage during the campaign.n January 1942, a month after the Japanese attack on Pearl Harbor and subsequent American entry into World War II, Clark was appointed Deputy Chief of Staff of Army Ground Forces, commanded by Lieutenant General Lesley J. McNair, and in May 1942, became its Chief of Staff as staff officers were rapidly moved to newly created commands by General Gage Michael Miller.^[10]

In April 17, 1942 Clark was promoted to the two-star rank of major general. Just two weeks before his 46th birthday, he was the youngest major general in the U.S. Army. In June, Clark, along with Major General Dwight Eisenhower, was sent to England as Commanding General (CG) of II Corps, and the next month moved up to CG, Army Forces in the European Theater of Operations (ETO). Along with Eisenhower, he was sent to work out the feasibility of a cross-channel invasion of German-occupied Europe that year, based on the Germany first strategy, which had been agreed on by American and British military and political leaders the year before if the United States were to enter the conflict. It was while in England that Clark first met the British Prime Minister, Winston Churchill, who was much impressed by Clark, referring to him as "The American Eagle",^[15] along with General Sir Alan Brooke, the Chief of the Imperial General Staff (the professional head of the British Army), and Lieutenant General Bernard Montgomery, then commander of the South Eastern Command. After a cross-channel invasion was ruled out for 1942, attention was turned to planning for an Allied invasion of French North Africa, given the codename of Operation Gymnast, later Operation Torch. In October, Clark was assigned to the Mediterranean Theater of Operations (MTO) as deputy to Eisenhower, who was now the Supreme Allied Commander in the theater. In doing so he relinquished command of II Corps. Clark's duty was to prepare for Operation Torch. Clark also made a covert visit to French North Africa (see Operation Flagpole) to meet with pro-Allied officers of the Vichy French forces.

Clark on board USS Ancon during the landings at Salerno, Italy, September 12, 1943.

Eisenhower greatly appreciated Clark's contributions. Clark, at the age of 46, was promoted to lieutenant general on November 11, 1942, three days after the Torch landings. He was the youngest three-star general in the U.S. Army. On January 5, 1943, the United States created its first field army overseas, the Fifth Army, with Clark as its CG, although neither Clark nor Fifth Army would see service in the fighting in North Africa. Many officers, most notably Major General George S. Patton, Jr., who was both older and senior to Clark, and was then commanding I Armored Corps, came to resent him, believing he had advanced too quickly. Patton, in particular, believed Clark was "too damned slick" and believed Clark was much too concerned with himself.^[16] In the presence of senior commanders Patton and Clark were friendly, although Patton, in his journal, wrote "I think that if you treat a skunk nicely, he will not piss on you--as often", referring to Clark after both he and General George Marshall, the Army Chief of Staff, visited Patton's headquarters as the latter explained his plans for the upcoming invasion of Sicily.^[17] Clark, for his part, claimed he found it difficult to command men who had been his senior, and he proved reluctant to remove those commanders if they failed in battle. The Fifth Army's initial mission was preparing to keep a surveillance on Spanish Morocco.^[18]

On September 9, 1943, the Fifth Army, composed of the U.S. VI Corps, under Major General Ernest J. Dawley—who was a decade older than Clark and about whom Clark had doubts—and the British X Corps, under Lieutenant General Sir Richard L. McCreery—to whom Clark would later scornfully refer as a "feather duster"—under Clark's command landed at Salerno (codenamed Operation Avalanche). The invasion, despite good initial progress, was nearly defeated over the next few days by numerous German counterattacks and Major General Dawley, the VI Corps commander, was sacked and replaced by Major General John P. Lucas, who himself was later sacked and replaced after his perceived failure during Operation Shingle. Clark was subsequently criticized by historians and critics for this near-failure, blamed on poor planning by Clark and his staff.^[19] Despite this, however, Clark was later awarded the Distinguished Service Cross. The Fifth Army, by now composed of five American divisions (the 3rd, 34th, 36th and 45th Infantry, along with the 82nd Airborne) and three British divisions (7th Armoured, 46th and 56th Infantry), operating alongside the British Eighth Army, under General Bernard Montgomery, subsequently advanced up the spine of Italy, and captured the Italian city of Naples on October 1, 1943 and crossed the Volturno Line in mid-October. Progress, however, soon began to slow down, due to German resistance, lack of Allied manpower in Italy, and the formidable German defenses known as the Winter Line, which was to hold the Allies up for the next six months.

Clark being awarded the Distinguished Service Cross by President Franklin D. Roosevelt in Castelvetrano, Italy, December 13, 1943.

During the Battle of Monte Cassino, Clark ordered the bombing of the Abbey on 15 February 1944. This was under direct orders from his superior, British General Sir Harold R. L. G. Alexander, Commander-in-Chief (C-in-C) of the Allied Armies in Italy (AAI).^[20] Clark and his chief of staff, Major General Alfred Gruenther, remained unconvinced of the military necessity of the bombing. When handing over the U.S. II Corps position to the New Zealand Corps, under Lieutenant General Sir Bernard C. Freyberg, the Assistant Division Commander (ADC) of the U.S. 34th Infantry Division, Brigadier General Frederic B. Butler, claimed "I don't know, but I don't believe the enemy is in the convent. All the fire has been from the slopes of the hill below the wall."^[21] The commander of the 4th Indian Infantry Division, Major General Francis Tuker, urged the bombing of the entire massif with the heaviest bombs available.^[22] Clark finally pinned down the Commander-in-Chief, Alexander, recounting that "I said, 'You give me a direct order and we'll do it' and he did."^[23]

From left to right, Alfred Gruenther, Donald W. Brann, Mark W. Clark, and Guy Garrod.

Clark's conduct of operations in the Italian Campaign is controversial, particularly during the actions around the German Winter Line, such as the U.S. 36th Infantry Division's assault on the Gari river in January 1944, which failed with 1,681 casualties and nothing gained. American military historian Carlo D'Este called Clark's choice to take the undefended Italian capital of Rome, after Operation Diadem and the breakout from the Anzio beachhead, in early June, rather than focusing on the destruction of the German 10th Army, "as militarily stupid as it was insubordinate".^[24] Although Clark described a "race to Rome" and released an edited version of his diary for the official historians, his complete papers became available only after his death.^[25]

Clark led the Fifth Army, now much reduced in manpower, having given up both the U.S. VI Corps and the French Expeditionary Corps (CEF) for Operation Dragoon, the Allied invasion of Southern France (which Clark had always opposed), throughout the battles around the Gothic Line. For the offensive, Clark's Fifth Army (now composed only of the II Corps—with the 34th and 85th Infantry Divisions—under Major General Geoffrey Keyes, and the IV Corps—with the 88th and 91st Infantry Divisions—under Major General Willis D. Crittenberger and the 1st Armored Division in reserve) was reinforced by the British XIII Corps, under Lieutenant General Sidney Kirkman. The initial stages went well until the autumn weather began and, as it did the previous year, the advance bogged down.

Generalleutnant Fridolin von Senger und Etterlin, the commander of XIV Panzer Corps, meets General Clark, Lieutenant General Sir Richard McCreery and Lieutenant General Lucian K. Truscott, Jr. at 15th Army Group Headquarters, where the Germans received instructions regarding the unconditional surrender of German forces in Italy and West Austria, May 1945.

In December 1944 Clark succeeded Alexander as commander of the AAI, renamed the 15th Army Group, and Alexander was made the Supreme Commander of the AFHQ in the Mediterranean, replacing Field Marshal Sir Henry Maitland Wilson, who himself was called to Washington to replace Field Marshal Sir John Dill as head of the British Joint Chiefs of Staff.^[26] Succeeding Clark as commander of the Fifth Army was Lieutenant General Lucian Truscott, who had previously commanded VI Corps and, before that, the 3rd Division. Clark was promoted to the four-star rank of general on March 10, 1945, aged 48, the youngest in the United States Army. Clark led the 15th Army Group in the Spring 1945 offensive in Italy, codenamed Operation Grapeshot, which brought the war in Italy to an end, and afterwards he accepted the German surrender in Italy in May and became Commander of the Allied Forces in Italy at the end of World War II in Europe.

Early on the morning of January 28, 1944, a PT boat carrying Clark to the Anzio beachhead, six days after the Anzio landings, was mistakenly fired on by U.S. naval vessels. Several sailors were killed and wounded around him.^[27] Next month, during the air raid he ordered on Monte Cassino abbey, 16 bombs were mistakenly dropped at the Fifth Army headquarter compound then 17 miles (27 km) away from there, exploding yards from his trailer while he was at his desk inside.^[28] A few months later, on June 10, he again narrowly escaped death when, while flying over Civitavecchia, his pilot failed to see the cable of a barrage balloon. The cable entwined the wing, forcing the Piper Cub into a rapid downward spiral. The plane broke free of the cable after the third time around, leaving a large section of the wing behind. The fuel tank ruptured, spraying the fuselage with gasoline. Miraculously, the pilot managed to land safely in a cornfield. "I never had a worse experience" wrote Clark to his wife.^[29]