US

In the 20th century these were the years that the USA wasn’t fighting a war somewhere;

1935–1936: No major foreign wars or invasions; Banana Wars mostly wrapped up.

1938: U.S. was not yet involved in WWII; military remained largely in training/preparation.

1977: After Vietnam withdrawal and before major Cold War interventions resumed.

1979: Slight pause between covert activity in Africa and full engagement in later Cold War interventions.

1980: No large-scale combat. However, note the failed Iran hostage rescue mission (Operation Eagle Claw).

Three years (max) in a century in which it is clear that no US military were killed in foreign excursions.