WASHINGTON — President Trump insists that choosing when to reopen the economy is “the biggest decision I’ve ever had to make,” and, in the short term, he is undoubtedly right. It is a perilous balance of public safety versus economic revival that would test any president, even one not consumed by a looming election.

But behind it is an even broader challenge: Whether an avowedly “America First” president who has always measured national strength by the size of the Pentagon budget, the number of ships in the Navy and improving the nuclear arsenal has now seen a peril that would lead him to drastically shift how he thinks about the nation’s security.

Another president might see this as the moment to gather nations together in a collective fight against a virus that leaps borders at astounding speed.

But, so far, Mr. Trump has shown little interest in collective action, apart from episodic telephone calls with allies. His announcement on Tuesday that he would withhold American funding from the World Health Organization — for making the same mistakes he did, underreacting to the coronavirus outbreak and praising Chinese “transparency” — suggests that, if anything, he is again determined to go it alone.