I did some research in the Internet regarding America's direct involvement and interventions in wars fought on American soil and some foreign lands. From what I've found out, the US of A has been constantly at war since 1622 until the present day. Check this out:

1622 – 1890 : Indian Wars

1689 – 1763 : Anglo – French Colonial Wars

1753 – 1763 : French – Indian War

1763 – 1775 : American Revolution

1775 – 1783 : War for Independence

1812 : War of 1812 (USA - Great Britain)

1845 – 1848 : Mexican – American War

1861 – 1865 : American Civil War

1898 – 1902 : Spanish – American War

1900 : China (Intervention in Boxer Rebellion)

1904 : Panama (US Military occupation)

1904 – 1906 : Morroco (US Marine interventions)

1907 – 1924 : Dominican Republic (US Military occupation)

1914 – 1917 : Mexico (US Marine intervention)

1915 – 1934 : Haiti (US Marine occupation)

1917 – 1918 : World War I

1918 – 1920 : Russia (Allied Force vs Bolsheviks)

1941 – 1945 : World War II

1947 – 1991 : Cold War:-

a) 1950 – 1953 Korean War

b) 1965 – 1975 Vietnam War

c) 1979 – 1989 Soviet - Afghanistan War

1948 : Lebanon (US Marine intervention)

1954 : Guatemala (CIA intervention)

1961 : Cuba (CIA intervention)

1965 : Dominican Republic (US Marine occupation)

1975 : Cambodia (US Marine intervention)

1983 : Grenada (US Marine intervention)

1986 : Libya (Bombing of Tripoli)

1989 – 1990 : Panama (Overthrown Noriega)

1991 : Gulf War (Operation Desert Storm)

1994 : Somalia

2001 – Present : War on Terror

a) 2001 : American – Afghanistan War

b) 2003 : Invasion of Iraq (Operation Iraqi Freedom)

c) Future : Iran, Syria, Libya, North Korea, Pakistan, the rest
of the world…