[Source:] On 11 April 1951, President Truman drafted an order to MacArthur, which was issued under Bradley's signature:
I deeply regret that it becomes my duty as President and Commander-in-Chief of the United States military forces to replace you as Supreme Commander, Allied Powers; Commander-in-Chief, United Nations Command; Commander-in-Chief, Far East; and Commanding General, U.S. Army, Far East.
become = 1. [no object, with complement] Begin to be:
I'm not that knowledgeable of US history, but when Truman (or Bradley) wrote this letter, did the US President already hold this duty to replace a General?
If so, then is this use of to become wrong? Such use wrongly means that Truman only began to hold this duty precisely at the moment that he (or Bradley) wrote this