Holden's character was a Canadian
Several references here mention the character as being American, and a character near the beginning asks him if the Americans have joined the war. Near the beginning of the film when he is talking to the command guy, he mentions he was a Sergeant in the Canadian Army and was selected for service due to being a tug Captain 10 years earlier.
Ignoring politics doesn't mean politics will ignore you.
-Pericles paraphrased in <100 characters