Title: List of battles with most United States military fatalities
Content:  single day battle in American history, if all engaged armies are considered, is the Battle of Antietam with 5,389 killed, including both United States and enemy soldiers (total casualties for both sided was 22,717 dead, wounded, or missing American and enemy soldiers September 17, 1862). The origins of the US military can be traced to the Americans' fight for independence from their former colonial power, Great Britain, in the American Revolutionary War (1775–1783). The three bloodiest conflicts have been American Civil War (1861–1865), World War I (1917–1918), and World War II (1941–45). Other significant conflicts involving the United States ordered by casualties include the Korean War (1950–1953), the Vietnam War (1964–1973), the War in Afghanistan (2001–present), and various conflicts in the Middle East. For most of its existence, America has been involved in one or another military conflict.
Title: List of battles with most United States military fatalities
Content:  This article lists battles and campaigns in which the number of US soldiers killed was higher than 1,000. The battles and campaigns that reached that number of deaths in the field are so far limited to the American Civil War, World War I, World War II, Korean War, and one campaign during the Vietnam War (the Tet Offensive from January 30 to September 23, 1968). The campaign that resulted in the most US military deaths was the Battle of Normandy (June 6 to August 25, 1944) in which 29,204 soldiers were killed fighting against Nazi Germany. The bloodiest single day in the history of the of the United States Military was June 6, 1944, with 2,500 soldiers killed during the Invasion of Normandy on D-Day. The second-highest single-day toll was the Battle of Antietam with 2,108 dead. The 
Title: List of battles with most United States military fatalities
Content:   This article contains the list of battles with most United States military fatalities, in terms of American deaths.
Title: Korean War
Content:   According to the data from the US Department of Defense, the US suffered 33,686 battle deaths, along with 2,830 non-battle deaths, and 17,730 other deaths during the Korean War. American combat casualties were over 90 percent of non-Korean UN losses. U.S. battle deaths were 8,516 up to their first engagement with the Chinese on 1 November 1950. The first four months of the Korean War, that is, the war prior to the Chinese intervention (which started near the end of October), were by far the bloodiest per day for the US forces as they engaged and destroyed the comparatively well-equipped KPA in intense fighting. American medical records show that from July to October 
Title: Korean War
Content:  the US Army sustained 31 percent of the combat deaths it ultimately incurred in the whole 37-month war. The U.S. spent US$30 billion in total on the war. Some 1,789,000 American soldiers served in the Korean War, accounting for 31 percent of the 5,720,000 Americans who served on active-duty worldwide from June 1950 to July 1953. South Korea reported some 137,899 military deaths and 24,495 missing. Deaths from the other non-American U.N. militaries totaled 3,730, with another 379 missing. Data from official Chinese sources reported that the PVA had suffered 114,000 battle deaths, 34,000 non-battle deaths, 340,000 wounded, and 7,600 missing during the war. 7,110 Chinese POWs were repatriated to China. In 2010, 
Title: Financial cost of the Iraq War
Content:  The US has lost a number of pieces of military equipment during the war. The following statistics are from the Center for American Progress; and they are only approximations that also include vehicles lost in non-combat-related accidents as of 2006.
Title: Pacific War
Content:  There were some 426,000 American casualties: 161,000 dead (including 111,914 in battle and 49,000 non-battle), 248,316 wounded, and 16,358 captured (not counting POWs who died). Material losses were 188+ warships including 5 battleships, 11 aircraft carriers, 25 cruisers, 84 destroyers and destroyer escorts, and 63 submarines, plus 21,255 aircraft. This gave the USN a 2-1 exchange ratio with the IJN in terms of ships and aircraft. The US protectorate in the Philippines suffered considerable losses. Military losses were 27,000 dead (including POWs), 75,000 living POWs, and an unknown number wounded, not counting irregulars that fought in the insurgency. Between 500,000 and 1,000,000 Filipino civilians died due to either war-related shortages, massacres, shelling, and bombing.
Title: Results of the War of 1812
Content:  During the war, roughly 15,000 American soldiers and sailors died, 3,721 in combat. The war cost the United States about $200 million. Neither the United States nor the United Kingdom gained any military advantage, but indirectly, the United States made some gains. A significant military development was the increased emphasis by General Winfield Scott on professionalism in the Army's officer corps, particularly the training of officers at the US Military Academy ("West Point"). The new professionalism would become apparent during the Mexican–American War (1846–1848). After the annexation of Texas by the United States, the term "Manifest Destiny" became a widely-used political term for those who propagated American expansionism and military pride. In a related development, the United States officially abandoned its reliance on the militia for its defense in favor of a standing army. Moreover, the United States Army 
Title: War
Content:  psychiatric disturbances in varying degrees." - 14–18: Understanding the Great War, Stéphane Audoin-Rouzeau, Annette Becker Additionally, it has been estimated anywhere from 18% to 54% of Vietnam war veterans suffered from posttraumatic stress disorder. Based on 1860 census figures, 8% of all white American males aged 13 to 43 died in the American Civil War, including about 6% in the North and approximately 18% in the South. The war remains the deadliest conflict in American history, resulting in the deaths of 620,000 military personnel. United States military casualties of war since 1775 have totaled over two million. Of the 60 million European military personnel who were mobilized in World War I, 8 million were killed, 7 million were 
Title: American Civil War
Content:  earliest to use industrial warfare. Railroads, the telegraph, steamships, the ironclad warship, and mass-produced weapons saw wide use. In total the war left between 620,000 and 750,000 soldiers dead, along with an undetermined number of civilian casualties. President Lincoln was assassinated just five days after Lee's surrender. The Civil War remains the deadliest military conflict in American history. "It was only as recently as the Vietnam War that the number of American deaths in foreign wars [combined] eclipsed the number who died in the Civil War." The technology and brutality of the Civil War foreshadowed the coming World Wars.

Question: what war did the us lose the most soldiers?
Answer: Vietnam War.