The militant political group Hezbollah and its allies forced the collapse of Lebanon's government while its prime minister sat in the White House, dealing a blow to Washington in its battle with Iran and Syria for influence in the Mideast.

Eleven ministers from Hezbollah and their allies pulled out of the 30-member coalition cabinet Wednesday after Prime Minister Saad Hariri refused to discredit a United Nations-backed court investigating the 2005 assassination of his father, former Prime Minister Rafik Hariri.

The...