A senior figure from a Syrian rebel group with links to al Qaida was allowed into the United States for a brief visit, raising questions about how much the Obama administration will compromise in the search for partners in the conflict. Labib al Nahhas, foreign affairs director for the Islamist fighting group Ahrar al Sham, spent a few days in Washington in December, according to four people with direct knowledge of the trip and who spoke on condition of anonymity. His previously undisclosed visit is a delicate matter for both sides – the conservative Salafist insurgents risk their credibility with...