U.S. Secretary of State John F. Kerry delivered an impassioned defense of diplomacy Monday, asserting that the nuclear deal he spearheaded with Iran had averted an almost-certain war and vowing to work “to the last moment” to achieve a lasting cease-fire in Syria.

In London to accept the Chatham House Prize, given by the prestigious think tank, Kerry said he would “rather be caught trying” to reach seemingly impossible diplomatic agreements than submit to the inevitability of war.

“Believe it or not, there’s nothing inevitable about conflicts raging in places like Syria, Yemen, Libya and South Sudan,” he said. “If war is a choice, then peace is also a choice.”

But after years of futile efforts to stop the conflict in Syria, the secretary also acknowledged just how difficult it will be for the region’s warring parties to end the violence in that ravaged nation.