There are only nineteen years since 1798 when the U.S. did not have armed forces engaged in military operations abroad, according to an updated tally from the Congressional Research Service. See Instances of Use of United States Armed Forces Abroad, 1798-2019, updated July 17, 2019.
The most recent year in which U.S. military forces were not used in a foreign conflict was 1979, according to the CRS. The CRS account does not reflect covert action, disaster relief, or training activities involving U.S. forces abroad.
Though there have only been 11 formal declarations of war, there have been hundreds of military actions including “extended military engagements that might be considered undeclared wars.”
“These cases vary greatly in size of operation, legal authorization, and significance,” CRS said. “Some actions were of short duration, and some lasted a number of years. In some examples, a military officer acted without authorization; some actions were conducted solely under the President’s powers as Chief Executive or Commander in Chief; other instances were authorized by Congress in some fashion.”