The last time they were used in the act of war? Hiroshima and Nagasaki.
But countries still test out their nuclear arms at certain bases. I recall hearing news of North Korea testing out their nukes:
North Korea was a Party to the Nuclear Non-Proliferation Treaty, but announced a withdrawal on January 10, 2003, after the United States accused it of having a secret uranium enrichment program and cut off energy assistance under the 1994 Agreed Framework. The last time they tested out their nukes was in 2006.
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The only nuclear weapons ever used were 'Little Boy', dropped on Hiroshima on the 6th of August 1945 and 'Fat Man' dropped on Nagasaki on the 9th by the USA.
No nuclear weapons were used in the korean war
The U.S used no nuclear weapons whatsoever on Vietnam.
The United States used the atomic bomb offensively during WWII. That is pretty close to nuclear weapons.
The only country that has ever used nuclear weapons in any war was the US, at the end of WW2.