(Ongoing wars indicated in bold and with orange bars.) Source: Associated Press
Article One, Section Eight of the Constitution gives the United States Congress the power to declare War". Historically wars have been either declared as "Formal" wars or authorized as "Authorized military engagements". In 1973 Congress further clarified their role in authorized armed conflict with the passing of the War Powers Resolution.
For the Philippine–American War, not included by the Associated Press in their tally above, official start and end dates used by some sources are June 2, 1899 – July 4, 1902, (three years and one month,) although resistance groups continued fighting until June 15, 1913.
The start date used by the Associated Press for U.S. engagement in the Persian Gulf War is January 17, 1991 (the start of its extensive aerial bombing campaign under the offensive known as Operation Desert Storm.) While the first US Combat Troops arrived in South Vietnam in 1965, by the end of 1964 over 23,000 American military personal were already present.
U.S. President Barack Obama pledged in 2009 and 2010 that the U.S. war in Iraq would end by the end of 2011 when all remaining U.S. troops would be withdrawn from Iraq. Then, in the Spring of 2011, Secretary of Defense Gates, went to Iraq, pleading with the government of Iraq, to let U.S. Forces remain past the 2011 deadline.