The U.S. military could destroy the Syrian air force, Gen. Martin Dempsey said in a Monday letter to Rep. Eliot Engel of New York, the top Democrat on the House Foreign Affairs Committee.
While that "would not be militarily decisive," Dempsey said, it would "commit us decisively to the conflict."
In addition, U.S. military involvement would essentially provide symptom relief rather than seek to remedy the warfare's root causes, said Dempsey, the highest-ranking U.S. military officer and President Obama's chief military adviser.
"In a variety of ways, the use of U.S. military force can change the military balance, but it cannot resolve the underlying and historic ethnic, religious and tribal issues that are fueling this conflict," he said in the letter, which can be found at tinyurl.com/UPI-Dempsey-letter-to-Engel.
Dempsey said the rebel groups Washington might wish to support with military involvement have not shown they will align with U.S. interests.
"Syria today is not about choosing between two sides but rather about choosing one among many sides," he said. "It is my belief that the side we choose must be ready to promote their interests and ours when the balance shifts in their favor. Today, they are not."
Syria's crisis is "tragic and complex," Dempsey said.
"It is a deeply rooted, long-term conflict among multiple factions, and violent struggles for power will continue after [Syrian President Bashar] Assad's rule ends," he said. "We should evaluate the effectiveness of limited military options in this context."
Engel said in response he was disappointed and unconvinced by Dempsey's letter.
"I remain deeply unsatisfied with our current strategy in Syria -- as we stand on the sidelines when the turmoil in that country continues to claim thousands of lives and sow instability throughout the region," Engel said in a statement Wednesday.
"I reject the notion that our involvement in Syria would simply constitute 'choosing sides' between one armed group and another," he said.
"Rather, our involvement represents a choice between hastening the end of the Assad regime or continuing to allow the cycle of violence, displacement and terror to continue unabated," he said.
Engel released the letter the same day Syrian rebel groups made fresh claims forces loyal to Assad carried out a "poisonous gas" attack near the capital Damascus that opposition groups claimed left more than 1,100 dead.
White House spokesman Josh Earnest said the Obama administration was "deeply concerned" about the reports and a senior U.S. official told The Wall Street Journal the administration had "strong indications" the regime used chemical weapons.
Syrian authorities denied using chemical weapons, accusing the opposition of fabricating claims or staging gas attacks.
Obama said a year ago Syrian use of chemical weapons would cross a "red line."
But the White House conclusion in June the Syrian army used chemical weapons did not bring about a noticeable shift in U.S. engagement.