LIBYA, WHICH has lived with civil war off and on since 2011, now is on the brink of joining Syria as the site of a major multinational conflict. Last Thursday, Turkish President Recep Tayyip Erdogan announced that he would ask his parliament as soon as this week to authorize the deployment of Turkish troops to defend Libya’s Tripoli-based government. The intervention is a response to the appearance of more than 1,000 Russian mercenaries in the ranks of rebel forces besieging the capital — something that has caused a shift in front lines for the first time in months.

Also pouring in resources to the insurgency led by warlord Khalifa Hifter are Egypt, the United Arab Emirates, France and Jordan, according to a recent U.N. report. Qatar has joined Turkey in backing the U.N.-recognized Government of National Accord (GNA). Both sides are now equipped with fleets of drones that have caused extensive civilian casualties. The escalation of fighting could touch off a new wave of refugees to Europe and allow a Libya-based Islamic State affiliate to gain ground.

All of this is happening in part because the United States has failed to exercise its influence with the combatants and their outside allies. Instead, the Trump administration has sent mixed signals. Officially, it supports the Tripoli government; but last April, President Trump took a phone call from Mr. Hifter, a onetime U.S. resident, and indicated support for his cause.

More recently, the administration has objected to the Russian intervention and tried to persuade both Mr. Hifter and the government of Vladi­mir Putin to agree to a negotiated settlement. Yet on Thursday Mr. Trump discussed Libya with Egyptian dictator Abdel Fatah al-Sissi, one of Hifter’s strongest supporters. An ambiguous White House statement said the two “rejected foreign exploitation and agreed that parties must take urgent steps to resolve the conflict before Libyans lose control to foreign actors.”

There’s little doubt that the foreign actor Egypt objects to is Turkey. Mr. Erdogan’s Islamist government is seen in Cairo as supporting Libya’s Islamist forces; Mr. Hifter, in contrast, aspires to be a secular dictator, like Mr. Sissi. In reality, as The Post’s Sudarsan Raghavan recently reported, Islamists are fighting on both sides, and their influence is limited.

Libya’s chaos began after the United States joined with European allies in helping to topple the dictatorship of Moammar Gaddafi in 2011, then made no serious effort to stabilize the country afterward. Years of painstaking U.N. mediation eventually produced the GNA from an amalgam of competing groups. But from a base in eastern Libya, Mr. Hifter has played spoiler, with help from his foreign sponsors.

Russia has played a particularly malevolent role. Mr. Putin, who strenuously objected to the overthrow of the Gaddafi regime, would like to restore Moscow’s influence in the country, much as he has in Syria. The mercenaries fighting for Mr. Hifter technically belong to a private company, the Wagner Group, but they do the Kremlin’s bidding. Legislation in Congress would sanction Russia for its Libya meddling; if the Trump administration genuinely wants to force a diplomatic solution, it will need such tools.

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