Vance Serchuk, a former foreign policy adviser to Sen. Joseph Lieberman, is a Council on Foreign Relations international affairs fellow, based in Tokyo. This is his first monthly column for The Post.

John Kerry will depart shortly on his first overseas trip as secretary of state, a major focus of which, he has said, will be the conflict in Syria. There is no crisis more deserving of the attention of America’s top diplomat, and Kerry deserves credit for immediately throwing himself into the fray.

In previewing his trip, however, Kerry has suggested that the key to ending the bloodshed in Syria is to change Bashar al-Assad’s “calculations” — a formulation he has invoked repeatedly in recent weeks — and that the path for doing so may run through Moscow.

This raises the specter that, rather than forging a new direction on Syria policy, the Obama administration is poised to repeat mistakes of the past.

The first of these has been to vest unrealistic hopes in the Russians. If the past two years should have taught anything, it is that the Kremlin is unlikely to help the United States orchestrate Assad’s exit.

This is not because of Russian arms sales or naval facilities in Syria, nor for any lack of U.S. engagement with Moscow. Rather, the Kremlin believes it has a broader interest in thwarting another U.S.-engineered regime change — seeing such interventions, stretching from Serbia to Libya, as a threat to international stability and as a precedent that could someday be used against itself.

More important, the Russians have less confidence than Washington does in their influence over Damascus. Even if Moscow were to pressure Assad, it is far from clear that would prompt him to consider leaving when countless other diplomatic and military setbacks haven’t, including the loss of Assad’s Turkish allies and the northern third of his country to rebels.

This points to a second, deeper problem with Kerry’s formulation. The United States has long staked its strategy on the hope that persuading Assad and the worst of his cronies to go would pave the way for a negotiated settlement between a unified Syrian opposition and remnants of the regime — avoiding an Iraq-like state collapse. But this notion of a “peaceful political transition” is increasingly questionable.

Rather than regime change without state collapse, the inverse is unfolding in Syria: the emergence of a failed state in which a contracted, consolidated Assad regime fights on — more sectarian, repressive and tightly aligned with Iran and Hezbollah.

Such a regime is less likely to be willing or able to negotiate its own end — regardless of whether Assad is its leader — and more likely to keep fighting, even if this means abandoning Damascus and establishing an Alawite rump state on the Mediterranean coast, protected by chemical weapons and militias sponsored by Iran’s Revolutionary Guards.

Meanwhile, on the other side of the conflict, al-Qaeda-linked extremists continue to gain influence by providing the help the West won’t — likewise dimming the chances of a negotiated peace.

Washington can, of course, keep hoping that Russia will flip, Assad will go and a deal will follow that saves the Syrian state — but we cannot count on it. Just as Assad and the Iranians have their Plan B, so must we.

The first element of such a strategy should be to recognize that, if any prospect remains to change the calculations of Assad and his gang, it will be because of bold leadership from Washington, not Moscow — specifically, the use of limited military force, such as airstrikes, to neutralize Assad’s airpower, protect civilians in liberated areas and underscore that the Syrian leader’s cause is hopeless.

Just as a diplomatic settlement was impossible in Bosnia until NATO airstrikes pushed Slobodan Milosevic to the negotiating table, the same may prove true for Syria.

Second, we need to accept the possibility that a negotiated settlement won’t be achievable and start working to mitigate the most sinister consequences of state collapse.

This leads to the question of U.S. support for the Syrian opposition. Proponents of arming the rebels — who, as of last fall, we now know, included Obama’s last secretary of state, defense secretary, CIA director and chairman of the Joint Chiefs — have argued that doing so could help tip the balance against Assad, empower moderates and build leverage with the opposition.

These arguments still hold. But there is another, more compelling reason now: Lethal assistance is our last, best tool to help determine whether the post-Assad vacuum is filled by a unified, military opposition that can maintain something resembling order — or a patchwork of ethnic and sectarian militias over which we have no influence.

Any hope for the former will require not just funneling weapons to guerrilla groups in the shadows but also a large-scale, transparent U.S.-sponsored effort to train, equip and mentor a new Syrian army.

Such a shift in strategy would run against the instincts of the Obama administration — its aversion to nation-building and military intervention, and its preference for letting others lead.

If John Kerry hopes to save Syria, the leader whose calculations he will need to change is not Vladimir Putin nor even Bashar al-Assad but the president of the United States.