He commanded the 32nd Division during the final year of World War I, and served in numerous other conflicts during his career.

[7] In 1905 he graduated from the United States Army War College, the same year he married, on August 16, to a woman named Margaret.

[6] As Acting Chief of Staff of the Pacific Division, he assisted with relief work after the 1906 San Francisco earthquake.

[7][3] In April 1917, the same month of the American entry into World War I, he was given the temporary rank of brigadier general and assigned to command the 57th Field Artillery Brigade of the 32nd Division at Camp MacArthur, Texas.

Only two months later, the 32nd Division was activated in July 1917 at Camp MacArthur, Waco, Texas of National Guard units from Wisconsin and Michigan.

Major General James Parker reassumed command on December 7, 1917, leading the unit into Alsace in May 1918, attacking 19 kilometres (12 mi) in seven days.

The division's shoulder patch, a line shot through with a red arrow, signifies its tenacity during World War I.

[11] It was the only American unit in General Charles Mangins famous 10th French Army, it fought in the Oise-Aisne offensive.

[12] The division fought continuously for 20 days during the Meuse–Argonne offensive and penetrated the last German defensive stronghold, the Kriemhilde Stellung, crossing the Meuse River.

[12] It was for the fighting in the Meuse-Argonne that Haan would later be awarded the Army Distinguished Service Medal, the citation for which reads: The President of the United States of America, authorized by Act of Congress, July 9, 1918, takes pleasure in presenting the Army Distinguished Service Medal to Major General William George Haan, United States Army, for exceptionally meritorious and distinguished services to the Government of the United States, in a duty of great responsibility during World War I.

General Haan, in Command of the 32d Division, took a prominent part in the Argonne-Meuse offensive and in the brilliant and successful attack against the Cote Dame Marie, covering several days, which deprived the enemy of the key point of the position.

There they occupied the center sector in the Colbenz bridgehead for four months, during which they held 400 square kilometers and 63 towns.

[6] In April 1919 he returned to the US, again in commamd of the 32nd Division, and after its inactivation again, he served as head of the War Plans Division at the War Department in Washington, D.C.. His permanent rank was advanced to major general in 1920, although he was forced to retire from the army in 1922, after over thirty years of continuous service.