If we are speaking about the United States, the USA has been a participant in wars dating back to WW1 in 1917. They were:
1. A short time in WW1 before Germany sued for peace;

2. 1941 to 1945 in WW2;

3. A participant in the UN declared war to defend South Korea from North Korea in 1950.

4. The Vietnam War that began in the middle 1960's to 1972.

5. Desert Storm War in 1991

6. Invasion of Iraq in the early 2000's

7. War in Afghanistan begun during Pres. Bush's Administration until now with Pres. Obama.