WASHINGTON — Donald J. Trump has stirred outrage and incredulity with a string of recent comments about Russia — from cyberhacking to sanctions. Yet nothing he has said has more practical significance than the suggestion that, should he win election in November, American allies would lose the protection of the United States unless they “pay” more for their defense. His warning is a reminder of how easily presidents can blunder their way into big trouble.

Mr. Trump is right that European military budgets are too low. A vast majority of NATO members are not close to the target of 2 percent of gross domestic product for military spending that the alliance reaffirmed at its summit meeting in Warsaw this month. President Obama himself has called the United States’ allies “free riders.”

Nor is the Republican presidential candidate completely wrong that NATO members have a little wiggle room in coming to one another’s defense. The tiny bit of choice they have has sometimes worried allied governments.

All the same, Mr. Trump clearly knows nothing about the allies that he specifically said the United States might not defend. Together with Poland, the three small Baltic States — Latvia, Lithuania and Estonia — have been among NATO’s star performers. Between 2010 and 2015, they increased their military spending by 45 percent.