A think tank report says that U.S. intervention in Syria involving on-the-ground forces could require between 200,000 and 300,000 troops and cost up to $300 billion per year to be executed properly.

While no one is advocating a strategy involving an invasion, the report from the Brookings Institution’s Saban Center for Middle East Policy highlights the difficulties of accomplishing the Obama administration’s goal of removing Syrian President Bashar Assad from power.

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The report lays out six ways that Assad’s ouster could occur, but it doesn't any of the options. They range from a diplomatic solution — which the Obama administration supports — to a “Libya-like air campaign that’s backed by Sen. John McCain John Sidney McCainCindy McCain joins board of Biden's presidential transition team Meet the first woman to run for president Jill Biden shuts down Jake Tapper's question about husband's 'occasional gaffe' MORE (R-Ariz.), to a full-on invasion that would require significant troop numbers."

“At the end of the day, however, removing Assad may not be doable at a price the United States is willing to pay,” the report states. “Although the Obama administration has for months called for Assad to go, every policy option to remove him is flawed, and some could even make the situation worse — seemingly a recipe for inaction.”

The authors say they examined a U.S. invasion because if Washington is determined to stop the slaughter of innocent civilians, “an invasion may well be the only way to do so — it is certainly the only way that would be guaranteed to do so.”

“An invasion and the early months of an occupation would require 200,000 to 300,000 troops to be done properly,” they wrote. “That alone should put the bill for a Syrian invasion at around $200 to $300 billion per year for as long as that number of troops would have to remain.

“If the United States has learned the lessons of Iraq, those troop numbers could be reduced fairly quickly, perhaps in a matter of months, though lesser numbers of troops would have to remain for years,” the report says.

One alternative would be a NATO-led invasion of Syria, ideally with participation from the Arab League, the authors write.

So far no one, including hawks like McCain, have called for the U.S. military to put commit ground forces.

But unlike Libya, where NATO-led airstrikes helped topple Libyan leader Moammar Gadhafi, the Brookings report says a similar campaign would be much more difficult in Syria. That’s because Syria’s military is much more capable and the opposition is more divided; a more urban terrain in Syria also complicates matters.

The report says the Obama administration is currently engaged in “coercive diplomacy,” using sanctions against Assad and providing nonmilitary support to the opposition. The analysts write that while this could prove successful, it could also lead to “a prolonged and bloody stalemate.”

A diplomatic solution, the report says, “rests on a key assumption: that reversing Russia’s protection of the Assad regime in the Security Council is actually possible.”

Russia and China vetoed a United Nations Security Council resolution last month that would have pushed for Assad to relinquish power.