WordNet
n. a local military action without declaration of war; against violators of international peace and order
Wikipedia
Police action in military/ security studies and international relations is a euphemism for a military action undertaken without a formal declaration of war.
Since World War II, formal declarations of war have been rare, especially actions conducted by developed nations in connection with the Cold War. Rather, nations involved in military conflict (especially the major-power nations) sometimes describe the conflict by fighting the war under the auspices of a "police action" to show that it is a limited military operation different from total war.
The earliest appearance of the phrase was in 1883, referring to attempts by Netherlands forces and English forces to liberate the 28-man crew, of the SS Nisero, who were held hostage. The Dutch term politionele acties (police actions) was used for this. It was also used to imply a formal claim of sovereignty by colonial powers, such as in the military actions of the Netherlands, United Kingdom, and other allies during the Indonesian National Revolution (1945–1949) and the Malayan Emergency (1948–1960).
Usage examples of "police action".
A few would have been shot by Burr-head and his crew, but most would have tried to flee at the first sign of serious police action.
The reporter, went on to say that this had been the second terrorist incident in Europe over the last couple of months, both of them failures due to adroit police action.
I had to keep him in sight, move when he moved, follow him and find his contacts and follow them through a city where police teemed and where police action against any of these men would snap the thread and smash the whole day down.
The film contained several scenes of complete sexual frankness, and although it had not been banned, few cinemas were willing to show films which depicted the various forms of the sex act in detail because of several instances of police action.