Question: The 100th Infantry Battalion combined with a regiment composed of men of Japanese ancestry who fought primarily in what war?

Context paragraphs:
148th Infantry Regiment (United States): The 1st Battalion 148th Infantry Regiment, Ohio Army National Guard, is a parent regiment under the U.S. Army Regimental System, with headquarters at Walbridge, Ohio.  It currently consists of the 1st Battalion, 148th Infantry Regiment, an Infantry Battalion of the 37th Infantry Brigade Combat team located throughout northwest Ohio.

138th Infantry Regiment (United States): The 138th Infantry Regiment is an infantry battalion in the Missouri National Guard.  Of the original regiment, only the 1st Battalion remains an active National Guard unit.  As of 2008, the 1st Battalion, 138th Infantry Regiment is a light infantry battalion currently assigned to the 110th Maneuver Enhancement Brigade headquartered in Kansas City, Missouri.

22nd Battalion (Australia): The 22nd Battalion was an infantry battalion of the Australian Army.  It was raised in 1915 as part of the Australian Imperial Force for service during World War I and formed part of the 6th Brigade, attached to the 2nd Division.  It fought during the Gallipoli campaign and on the Western Front before being disbanded in 1919.  In 1921, it was re-raised as a part-time unit and was merged with the 29th Battalion in 1930 to form the "29th/22nd Infantry Battalion".  Split in August 1939 and known as the "22nd Battalion (South Gippsland Regiment)", the battalion saw action against the Japanese during World War II in the Huon Peninsula and New Britain campaigns.  The battalion was disbanded in June 1946.  It was re-established as part of the Royal Victoria Regiment in the mid-1960s as a remote area battalion within the Citizens Military Force, and later became a part of the 8th/7th Battalion, Royal Victoria Regiment.

442nd Infantry Regiment (United States): The 442nd Regimental Combat Team is an infantry regiment of the United States Army, part of the Army Reserve.  The regiment was a fighting unit composed almost entirely of American soldiers of Japanese ancestry (mostly from Hawaii) who fought in World War II.  Most of the families of mainland Japanese Americans were confined to internment camps in the United States interior.  Beginning in 1944, the regiment fought primarily in Europe during World War II, in particular Italy, southern France, and Germany.

100th Infantry Battalion (United States): The 100th Infantry Battalion is the only infantry unit in the United States Army Reserve.  In World War II, the then-primarily Nisei battalion was composed largely of former members of the Hawaii Army National Guard.  The 100th saw heavy combat during World War II before and after combining with the 442nd Infantry Regiment, another mostly Nisei military unit, into a single fighting combat team.  Based at Fort Shafter, Honolulu, Hawaii, the 100th Battalion has reservists from Hawaii, American Samoa, Guam and Saipan, and has deployed to Iraq and Kuwait.  The unit was unofficially nicknamed the "Purple Heart Battalion", with the motto "Remember Pearl Harbor".

Viru Infantry Battalion: The Viru Infantry Battalion (Estonian: "Viru jalaväepataljon" ) is a battalion of the Estonian Land Forces.  It is a part of the 1st Infantry Brigade and its primary task is to train conscription-based infantry and anti-tank units.  The battalion has been previously known as the 4th Infantry Regiment and as the 4th Single Infantry Battalion.  The battalion is currently based at Jõhvi and is commanded by Lieutenant Colonel Arno Kruusmann.

Battle of Slater's Knoll: The Battle of Slater's Knoll (28 March – 6 April 1945) was a battle during the Second World War fought between Australian and Japanese forces on Bougainville Island.  Part of the Bougainville campaign, the battle occurred as a force of about 3,300 Japanese from the Japanese 6th Division supported by a sizeable concentration of artillery launched a counterattack against the main Australian offensive which had been pushing south towards Buin, concentrating their attacks on Slater's Knoll near the Puriata River.  The Australian troops belonged to the 7th Brigade, with the 25th Infantry Battalion being the most heavily engaged, although the 9th Infantry Battalion and the 61st Infantry Battalion also took part in the fighting.

37th Field Artillery Regiment: The 37th Field Artillery Regiment is a field artillery regiment of the United States Army, and parent regiment under the U.S. Army Regimental System.  The regiment was first constituted 5 July 1918 in the National Army.  The regiment served with the 10th Division during World War I, and the 2nd Infantry Division during World War II.  Elements of the regiment have served with the 2nd Infantry Division, 6th Infantry Division, 79th Infantry Division, and 172nd Infantry Brigade, among other units.  Two battalions of the regiment are currently active, the 1st Battalion, 37th Field Artillery is the direct support cannon battalion, in support of the 1st Stryker Brigade Combat Team, 2nd Infantry Division and the 6th Battalion, 37th Field Artillery is a Multiple Launch Rocket System battalion in the 210th Field Artillery Brigade.

28th Battalion (Australia): The 28th Battalion was an infantry battalion of the Australian Army.  It was raised in early 1915 as part of the First Australian Imperial Force for service during the First World War and formed part of the 7th Brigade, attached to the 2nd Division.  It fought during the final stages of the Gallipoli campaign in late 1915 and then on the Western Front between 1916 and 1918.  At the end of the war, the 28th was disbanded in 1919 but was re-raised in 1921, as a part-time unit based in Western Australia.  During the Second World War, the 28th undertook defensive duties in Australia for the majority of the conflict, before seeing action against the Japanese in the New Britain campaign in 1944–45.  The battalion was disbanded in March 1946 but was re-formed in 1948 as an amalgamated unit with the 16th Battalion, before being unlinked in 1952 and re-raised as a full battalion following the reintroduction of national service.  It remained on the Australian Army's order of battle until 1960 when it was subsumed into the Royal Western Australia Regiment, but was later re-raised in 1966 as a remote area infantry battalion.  In 1977, the 28th was reduced to an independent rifle company, and in 1987 was amalgamated into the 11th/28th Battalion, Royal Western Australia Regiment.

4th Battalion 17th Infantry Regiment: The 4th Battalion 17th Infantry Regiment is a United States Army infantry battalion.  An earlier regiment designated the 17th Infantry Regiment was organized on January 11, 1812, but was consolidated with four other regiments as the 3rd Infantry in the post-war reorganization of the army following the War of 1812, due to the shattering losses it sustained at the River Raisin.  The current battalion was constituted within the 17th Regiment of Infantry on May 3, 1861.

Supporting paragraphs: 100th Infantry Battalion (United States) 442nd Infantry Regiment (United States)
Answer: World War II