By sending troops to Venezuela to shore up dictator Nicolás Maduro, Russia’s Vladimir Putin has crossed yet another line — the Monroe Doctrine, practically the oldest principle of US foreign policy.

Declared by President James Monroe back in 1823 to block European colonialism in the New World, the doctrine declares that interference by Old World nations in the Western Hemisphere is “the manifestation of an unfriendly disposition toward the United States.”

The Russian soldiers won’t make much difference: Indeed, they’ll add fuel to the burning popular rage against Maduro — who, with predecessor Hugo Chavez, has led Venezuela to disaster.

Yet Moscow’s move is a blatant challenge to Washington. Unanswered, it will make not just President Trump but America look weak.

Trump warned midweek that “Russia has to get out” of Venezuela and that “all options” are on the table for making it happen. National Security Adviser John Bolton followed up Friday by declaring such “provocative actions” to be “a direct threat to international peace and security in the region” and vowing “to defend and protect the interests of the United States and those of our partners in the Western Hemisphere.”

Moscow doubled down, saying its forces would stay “for as long as needed, and as long as the government of Venezuela needs them.”

This strengthens the case for the approach suggested by veteran diplomat Roger Noriega, namely American military support for a Venezuelan-led liberation force loyal to Interim President Juan Guaidó.

Most of the hemisphere, and most democracies outside it, recognize Guaidó as Venezuela’s legitimate leader. Maduro, meanwhile, has lost the support even of the nation’s poor, who’d been the bulwark of the regime.

Power outages are now common, and that only caps ever-growing shortages of food and basic medicines. By UN estimates, Reuters reports, a quarter of the populace needs humanitarian aid.

Aid that Maduro won’t let cross the borders or enter the ports: He’d rather see mass starvation.

Guaidó has called for a nationwide mass protest on Monday. Maybe that will trigger the start of mass military defections or prompt commanders to abandon a leader who has led the country to ruin.

Maduro’s rapid ouster would be a sufficient slap to Putin. One way or another, however, Washington has to hit him hard for this latest provocation.