he unrelenting global troubles confronting Barack Obama are about to converge on him all at once, providing a stern test of leadership for a first-year president who has pledged to "change the world."

In a span of four days, Obama will plunge into the politics of the United Nations and host a summit in Pittsburgh on the world's wobbling economy. The international stage is coming to him, and no one standing on it with him will have higher stakes.

Obama is under pressure to push along stalled Mideast peace, prove the United States is serious about climate change and rally allies against the nuclear threats of Iran and North Korea. Restless leaders in Europe and elsewhere are pressing Obama to reform risky U.S. financial behavior and get Congress on board.