Syria has proved to be President Obama’s greatest foreign policy failure. The president’s mixed signals and timidity early in the conflict was a missed opportunity for the United States to have influence over its outcome. Now the strategic void has allowed Russia and President Vladimir Putin to exert control over how the conflict plays out, without any clear indication where the unpredictable Mr. Putin plans to take things.

When this conflict began in 2011 after Syrian President Bashar al-Assad ordered his military to fire on pro-democracy protesters, President Obama made the proclamation that Mr. Assad had to go. Implicit in that position was that the United States would be ready to lend its backing. However, when the United States had the opportunity to arm and train rebel forces that had the goal of replacing Mr. Assad's regime with a secular, democratic government, tolerant of human rights, the president balked.

Cautious to a fault, President Obama did not want his administration “owning” the conflict if things went badly, or if arms fell into the wrong hands. In time, Jihadist rebel extremists, more ruthless and with the intent of displacing Assad’s brutality and intolerance with their own, came to dominate the fight against Assad and gain territory.

Worse yet, what is now known as the Islamic State has come to control swaths of territory in both Syria and neighboring Iraq. Its victories and cruelty toward nonbelievers of every stripe are attracting warriors from across the globe, sold on its twisted vision of Islamic glory.

In Syria, meanwhile, as estimated 250,000 have died, and as much as half the population has been displaced. A flood of refugees to neighboring Europe is causing a crisis there.

It is hard to imagine President Obama owning a worse outcome if he had acted forthrightly at the start.

The administration’s attempts at playing catch up have proved half-hearted and ineffective. U.S. air attacks have degraded IS forces to some degree and led to small victories in rolling back the Islamic State’s territorial gains, particularly by the Kurds. Largely, however, the Islamic State has solidified its presence and is adding more troops than it is losing. Belated efforts to train the right rebels have proved insufficient and wholly ineffective.

Now comes President Putin’s gambit. He has sent an estimated 2,000 men, attack jets, helicopters and armored vehicles in support of Russia’s long-time ally, President Assad, and the one-quarter of territory he still controls in Western Syria, aided by Hezbollah forces backed by Iran.

What is Mr. Putin’s game? He claims Russia will take the fight to the Islamic State in a way the West has been unwilling to.

However, it is highly unlikely the Russian president has any intent to introduce the massive military intervention that would be necessary to be a game changer. Russians remember Afghanistan and have no interest in reliving that debacle.

If the intent is to save President Assad, it only assures the civil war will drag on. Sunni Muslims, which account for 70 percent of the population, will never accept a ceasefire that keeps Mr. Assad’s Alawite sect in power. The poison gas, barrel bombs and starvation tactics employed by the Assad regime to retain control of government has only increased the hatred toward him manyfold.

In return for his nation’s military intervention, Mr. Putin should be demanding that Mr. Assad accept a plan to step aside, flee to a safe haven, and allow transition to a broad, coalition government. In such a scenario, the United States and its allies could coordinate military and diplomatic strategies with Russia to make sure the Islamic State and other jihadist groups have no role in a post-Assad government.

This means the best option for President Obama and his secretary of state, John Kerry, is to engage President Putin. That is far from ideal, but it is the place where the past missteps have led the administration.

Meanwhile, with the U.S. flying missions against the Islamic State and Russian air power propping up Assad, there is the potential for a mistake in which the two military forces engage. More reason to keep the lines of communication open with the Kremlin.

The United States cannot disengage and let Russia take the fight. That would betray those who believed in President Obama’s stated commitment to the overthrow of President Assad. It would also strengthen President Putin’s hand in the region.

It’s a mess, one the administration helped create and must now figure out.