Answer:
The conflict in Korea was called a "police action" because the US never actually declared war, but instead acted under the auspices of the United Nations.
Step-by-step explanation:
When North Korea invaded South Korea in 1950, under the auspices of the United Nations, the United States sponsored a police action. It was a war in all but name. The goal of the war was to prevent North Korea from imposing its communist rule on South Korea. It was called a police action as President Truman never sought a formal declaration of war from Congress, officially.
At a time when the world was still reeling from the second world war, this military involvement was very unpopular. Neither side was winning, and there were too many deaths.