That followed a statement on Friday by John R. Bolton, Mr. Trump’s national security adviser, who said that the arrival of roughly 100 Russian military “advisers” in the country, along with new arms, posed “a direct threat to international peace and security in the region.” Mr. Bolton also warned that no country should enter the Western Hemisphere “with the intent of establishing or expanding military operations.”

Such declarations seem to set up a test: Would Venezuela be the place where Mr. Trump, who has often seemed willing to tolerate Mr. Putin’s most audacious provocations, finally draws his own red line? And, if so, does he have a plan to enforce it?

It is notable that Mr. Bolton, not Mr. Trump himself, made the Friday announcement, with its echoes of the 1823 Monroe Doctrine, the first time the United States issued similar warnings telling foreign powers not to intervene in the Western Hemisphere. In offering the unvarnished declaration on Monday — and while Mr. Trump focuses on the southern border — Mr. Pompeo said he was merely repeating the president’s sentiments on Venezuela.

Yet Mr. Maduro is digging in — despite the economic sanctions against his government and the recognition of Mr. Guaidó as interim president by the United States and more than 50 other nations. And Mr. Putin may well see an opportunity to replay Syria in Venezuela, propping up another leader that many American officials said had to go — Mr. Maduro instead of President Bashar al-Assad of Syria, this time — and frustrating Washington’s regional goals.

The administration so far has been quite cautious when it comes to threatening military action in Venezuela. While there have been ritual reminders that “all options are on the table,” there is no indication that any military intervention — which has a long and unhappy history in Latin America — is being seriously contemplated.