Military action by the UN in Korea in 1950 could have been vetoed but the Soviets in the Security Council, but the Soviets had walked out earlier in protest over the UN/Security Council not recognizing the (then new) Communist China over Nationalist China (Taiwan).

Since the Soviets were boycotting the Security Council, and the Soviets were one of the parties that had absolute veto power, the UN Security Council authorizing UN military intervention in Korea easily passed by the remaining Security Council members.

It would be interesting to speculate what level of international military cooperation would the US have been able to obtain if the resolution had been vetoed.  I don't think that the lack of the resolution would have stopped US involvement.

Certainly a tactical failure on the Soviet side.

I think that many of the nations involved would have been there anyway...fighting the commies was very popular in those days.