Retired Marine Corps general James Mattis cautioned American lawmakers on Saturday against intervening in Syria’s civil war without clearly defining what they hope to achieve. “The desire to do something, the intention to do good, cannot take the place of pragmatic ‘what is possible?'” he said, warning that American involvement would be far from easy.

Speaking at an Aspen Security Forum in Colorado, Mattis, who retired last month as head of United States Central Command which is responsible for operations in the Middle East, said, “We have no moral obligation to do the impossible” — although he believed imposing a no-fly zone was impossible, if at a high cost and perhaps to little effect. “The killing will go on on the ground because they’re not using aircraft to do most of the killing already,” he said.

The United States imposed a no-fly zone over Libya in cooperation with Arab and NATO allies in 2011 when that country’s strongman, Muammar Gaddafi, tried to suppress a popular uprising against his regime. Several American lawmakers have called on the administration to do so in Syria as well to prevent forces that are loyal to President Bashar Assad from attacking rebels from the air.

Last year, America’s top military officer, General Martin Dempsey, seemed reluctant, telling CNN, “It’s a different challenge” from Libya, “geographically. It’s a different challenge in terms of the capability of the Syrian military.”

Mattis was also skeptical of indiscriminately arming Syrian rebels. “If we’re going to do it, let’s not go half way,” he said. Besides sending weapons, America should train opposition fighters if it wants to have a real influence in the outcome of a civil war that has lasted more than two years already.

President Barack Obama announced last month that the United States would start providing small arms to the rebels but Congress, worried about the radicalization of the opposition movement and reluctant to involve the country in what seems increasingly a sectarian conflict, may thwart such efforts. The United Kingdom has abandoned its own plans to send weapons into Syria for similar reasons.

Western allies in the region, notably Qatar and Saudi Arabia, have armed the rebels, possibly in collusion with nearby Jordan and Turkey, hoping that they will topple Assad who is the only Arab ally of their nemesis Iran.

Mattis admitted that Assad’s fall would be Iran’s “biggest strategic setback in 25 years,” denying it access to the Mediterranean Sea as well as its proxy in Lebanon, the militant organization Hezbollah — which also backs the regime in Damascus. Their support for Assad is critical, he said. “Bashar Assas had more reliable, committed allies than the opposition.”