More at The New York TimesWASHINGTON — For five years, President Obama has consciously recast how America engages with the world’s toughest customers. But with Russia poised to annex Crimea after Sunday’s referendum, with a mounting threat to the rest of Ukraine and with the carnage in Syria accelerating, Mr. Obama’s strategy is now under greater stress than at any time in his presidency.In his first term, the White House described its approach as the “light footprint”: “Dumb wars” of occupation — how Mr. Obama once termed Iraq — were out. Drone strikes, cyberattacks and Special Operations raids that made use of America’s technological superiority were the new, quick-and-dirty expression of military and covert power. When he did agree to have American forces join the bombing of Libya in 2011, Mr. Obama insisted that NATO and Arab states “put skin in the game,” a phrase he vastly prefers to “leading from behind.”As he learned to play the long game, the Treasury Department became Mr. Obama’s favorite noncombatant command. It refined the art of the economic squeeze on Iran, eventually forcing the mullahs to the negotiating table.But so far those tools — or even the threat of them — have proved frustratingly ineffective in the most recent crises.