A state of war exists when the armed forces of two countries engage intentionally in warlike acts against each other. Doesn't have to be formally declared. Hitler never declared war on anyone, just went in. We Brits, of course, did it properly and officially after giving him an ultimatum. The Japs actually intended to declare war on the US before Pearl Harbour but their embassy cocked up the timing.
There were Brit VCs in the Indonesian confrontation and Aussie ones in Vietnam.