The Allied onslaught on the Saint-Mihiel salient allowed the Americans to assemble their forces on the Western Front. The U.S. First Army won despite its lack of tactical expertise and comprehensive offensive plan. The combat was notable for the first major use of the U.S. Army Air Service (precursor to the U.S. Air Force) led by William "Billy" Mitchell and General Patton's aggressive tank attacks from the front lines, unlike many other officers during the war. 

General John Pershing, the American commander-in-chief in France, had resisted using his divisions piecemeal to support French and British operations, wanting to build a distinct U.S. army. He first used the U.S. First Army in battle on 12 September during the Saint-Mihiel salient attack. Two "super" corps with three offensive and one reserve divisions would lead the US assault. Two lesser French corps supported the western salient. General Erich von Ludendorff, short on men and knowing of the Allied attack, withdrew from the salient to a shorter, more easily defended line to the rear. Allies attacked as Germans retreated. The invading Americans took advantage of the Germans' lack of firepower to maintain the front line. Pershing ordered his commanders to push faster after being surprised by the initial American attack's simplicity. Lead units of the U.S. First Army encountered Allied soldiers approaching from the west by 13 September. Allied forces took the salient three days later, ending the offensive. Pershing sent his forces west to join the Meuse-Argonne attack. U.S.: 7,000; German: 17,500, including 10,000 seized.

It had been about four years since the war had begun by MAY of 1918. France, Great Britain, Russia, Italy, and Japan, known as the Allies, were always at war with Germany, Austria-Hungary, and Turkey, wasting generations of young men on bloody, ultimately pointless battles.

The Allies were relieved to hear that the United States had declared war on Germany a year earlier. But, onlookers still had grounds for tempered hope. Though America's industrial might and labor pool ultimately proved decisive, the country's armed forces were weak and unproven. Further, it would take months, if not years, to bring it to bear. Although small in scale, the May 28, 1918 U.S. victory at Cantigny demonstrated to both enemies and allies that the United States' armed forces were strong, well-led, and unyielding. Despite its best intentions, the United States was unprepared to combat in World War One. 52 years had passed since the end of the American Civil War. The Spanish-American War, the Philippine Insurrection, and other wars against Native American tribes in the southwest were among the lesser conflicts that had given the American military some experience in the ensuing decades. In comparison to the millions that France, Britain, Russia, and Germany had at their disposal, the total size of its standing army in 1917 was only around 130,000. 

More than three million men will soon be enlisted in the United States armed forces after the Selective Service Act was passed on May 18, 1917. Major General John J. "Black Jack" Pershing, a veteran of the frontier wars, would be tasked with heading the nucleus of the American Expeditionary Forces (AEF). It would be a tall order for Pershing to train such a large number of untrained "citizen troops" to fight against the battle-hardened Germans. Pershing's plan was to train soldiers for combat in the 20th century by establishing an operations and training staff. The general's focus was on avoiding the kind of disasters that had befallen British and French forces at Ypres, Verdun, and the Somme, so he studied the combat there with great care. It was not his intention to lead the AEF indiscriminately into the blood and filth of trench combat. Instead, he pushed the idea of "mobile operations" ideology. 

After that, his new army wouldn't take shape until the end of 1917. Moreover, additional education in France is required. Pershing had over six divisions, totaling 325,000 soldiers, in Europe by early 1918, and more were arriving every week. He was anxious to show the Allies and the Germans what his new army could do. Cantigny, a tiny town in northern France on the banks of the Somme River, provided the perfect setting.

The town of Cantigny had been captured by the Germans during their March Spring Offensive, which had resulted in a salient or bulge in the Allied defenses. For Pershing, this was a chance to draw some of the AEF into the fight. Under Pershing's instructions, U.S. Army 1st Infantry Division commander Major General Robert L. Bullard assigned Colonel Hanson E. Ely and the 28th Infantry Regiment to attack the German position and retake Cantigny.

The fighting started at 7 a.m. on May 28. Ely, a 49-year-old former West Point football player who stands at six feet tall, led 4,000 troops forward in the first American offensive of the war after nearly two hours of pre-dawn artillery bombardment by French batteries.French tanks backed up Ely as he and three battalions, protected by machine gun companies on either wing, charged the outer German defenses. Is there any hope for the US greenhorns when up against a seasoned foe? There was no definitive answer available at the time.

Rifle and machine gun fire, hand grenades, flamethrowers, and furious hand-to-hand fighting characterized the close action that lasted for an hour before the Germans finally withdrew. The town of Cantigny was taken by US troops (no longer in green) and 200 enemy soldiers were captured. The plan's creator, George C. Marshall, a relatively unknown colonel at the time, was impressed by the successful outcome. "The success of this part of the operation was so complete, and the list of victims so limited, that everyone was thrilled and delighted," stated the future U.S. Chief of Staff.  Yet, the combat was far from done for these newly seasoned American soldiers. Alexander von Ludendorff, the German supreme commander, saw the symbolic value of an American triumph and ordered the American Expeditionary Force (AEF) routed from Cantigny. Seeing that all eyes were on his force, Pershing gave the order to hold the hamlet at all costs.

Cantigny was bombarded for hours by General Oskar von Hutier and the German 82nd Reserve Division. By dusk, the ground assault began. The following two days would consist of nonstop rocket fire and infantry assaults against American positions.

After "1,603 losses, including 199 killed," by the morning of May 30 Ely and his 28th Infantry still held the town. The opening action and the counter attacks had cost Hutier's division almost 1,400 men. Cantigny was a minor skirmish when compared to the massive engagements fought on the Western Front. Nonetheless, in terms of forces committed and casualties sustained, its strategic significance surpassed its size. The American army of conscripts and volunteers had gone on the offensive against a seasoned adversary and withstood sustained counterattacks because of their preparation for modern warfare. From Major General Pershing to Colonel Hanson Ely at the brigade level, the men at Cantigny were directed by trained experts. Also, the American soldier demonstrated that he was willing and able to take on the combat and win.

On July 18, 1918, the American Expeditionary Forces (AEF) led by General John J. "Black Jack" Pershing engaged in one of their earliest battles of the war. Initiated by a German offensive against the AEF (the newest soldiers on the front) on 15 July, the engagement was a part of the Second Battle of the Marne and occurred during World War I.

Under the general supervision of Allied généralissime Ferdinand Foch, French (some of them colonial) and American forces between Fontenoy and Château-Thierry launched a counter-assault against the German fortifications on the morning of 18 July 1918. This was the first attack on a 25-mile-wide front in nearly a year. A majority of the fighting for the areas near Soissons and Château-Thierry was done by the US troops. The allies were able to keep their intentions secret, and their 04:45 attack caught the Germans off guard when troops went "Over the Top" without a prior artillery bombardment, instead moving closely behind a rolling barrage that had begun with remarkable synchronicity. At some point, the opposing assaults (lines) inter-penetrated, and independent American units kept fighting despite technically being behind enemy lines.