They know it the same way they know everything else that they are ordered to do – they are told by their superiors who were told by their superiors, etc?
Sounds like a pretty odd question unless I’m missing something very obvious.
In any event there are of course stories or isolated troops (solo or more) who didn’t know the war was over and kept on fighting and hiding, sometimes for decades. Japanese cases after WWII are notorious and easily googled.
Nowadays this would be passed down the chain of command and broadcast openly to everyone and people would be able to heard about in on radio and TV and on the internet. In the past it was a real issue that armies often didn’t know for some time when wars started and ended, because communication was so slow.
This is why in some cases armistices were agreed not to go into effect immediately, but at a certain date and time. The armistice that ended the fighting in WWI was famously designed to go into effect on the 11the hour of the 11th day of the 11th month of 1918 and fighting continued up until that very point.
Several Japanese soldiers held out for years and decades after the WWII fighting what they thought was a continuing guerilla campaign.
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