Kofi Annan, who's been assigned the hopeless task of resolving the Syrian crisis by diplomacy, is calling for "concerned nations" — including Russia and Iran — to confer this week in Geneva.

I sympathize with Annan, now the special Syria envoy of the United Nations and the Arab League. The Bosnian and Rwandan genocides happened while he was head of U.N. peacekeeping, and he desperately wants to prevent further slaughter in Syria. "The longer we wait, the darker Syria's future becomes," he rightly said last week, in a plea for key countries to develop a peace plan.

But here's the sad truth: Even though an unchecked civil war will devastate civilians, attract radical jihadis, and destabilize the entire region, diplomacy won't prevent this. It's time for a reality check on what the West can and can't do to curb the killing — and to prevent a new failed Islamist state.

First, let's dispense with the Annan Plan, which calls for a cease-fire, an end to violence, and the right of the opposition to protest freely, in Arab Spring fashion. This won't work because it would spell the end of the regime of Syrian President Bashar al-Assad, and he will never accept it. As for the opposition, some leaders might have bought into negotiations early on, but no more; they won't sit down with a mass murderer.

Nor should anyone delude himself about easing Assad out of power — an option Europeans and the Obama team once hoped for. Russian officials bluntly reject any such plan. Last week, Russian Foreign Minister Sergey Lavrov told the radio station Echo Moskvy: "This scheme is, unequivocally, unworkable from the very beginning. Because he [Assad] won't go." Translation: We won't push him to go.

The issue is not Assad himself, but the survival of the regime, which is Russia's last Mideast ally. If Assad goes, his family cabal and the regime based on his Alawite (Shiite) sect would be in deep jeopardy. This is unacceptable to Russian President Vladimir Putin, who detests revolutions from below.

Finally, no one should pretend it's still possible to stop the flow of arms to Syrian combatants. "It is far too late to choose between nonviolence and militarization," rightly notes Steven Heydemann, a Syria expert at the U.S. Institute for Peace.

The Iranians are arming their Syrian ally, and the Russians don't hide the fact that they're bolstering Syrian air defenses and refurbishing Syrian attack helicopters.

Meantime, opposition fighters inside Syria — most of whom took up arms only after the regime started killing civilians — are buying weapons with funds from Syrians abroad and from Arab Gulf countries.

So what, if anything, should the United States do?

What we should not do, despite Sen. John McCain's pleas, is establish no-fly zones protected by U.S. air power. The Arizona Republican neglects to say that setting up such zones would require U.S. warplanes to bomb Damascus in order to take out Syrian air defenses that are supplied by Moscow. Washington would have no backing from the U.N. Security Council, and no likely support from NATO. Even Mitt Romney — despite his constant critiques of President Obama's foreign policy — rejects this option.

But that does not mean we should do nothing, even though this is the position of many on both sides of the aisle.

A long, drawn-out sectarian civil war in Syria that pits rebellious Sunnis against Assad's Shiite sect will destabilize the region — and provide a new haven for Islamist jihadis. If funding and arming the rebels is left entirely to Saudi Arabia and Qatar, the opposition will become dominated by Islamists and will continue to fragment.

Here is where the United States can play a critical role.

Syrian activist Amr Al-Azm, a history professor at Shawnee State University, says Washington should be far more active in influencing which rebel groups get funding. "Right now you have a vacuum and anyone can jump in and set themselves up as warlords," says Al-Azm, the son of the famous Syrian philosopher and fighter for intellectual freedom Sadiq Al-Azm.

Al-Azm, and other non-Islamist activists, would like to see Washington channel funds from its allies to responsible fighting groups within Syria, and help them set up organizational structures — like provincial military councils.

The issue is not so much who buys the weapons, says Al-Azm, but "how to get some control over what is flowing in and how [the weapons] are used."

This will require Washington to get our Arab allies to agree on a strategy for arming the rebels, who need help with intelligence and communications, along with antitank and antiaircraft weapons. It also means the CIA will need far better intelligence about what is going on inside Syria.

A more proactive strategy might convince Syrians — and Putin — that Washington is serious about wanting Assad gone, which could lead to more and bigger defections within Syria's military. "Putin understands the West has no stomach for a fight," says Al-Azm. "If the West puts its feet down, Putin might think differently."

The Annan plan was Plan A, and it's over. It's time for the White House to adopt Plan B.

E-mail Trudy Rubin at trubin@phillynews.com.