DUBAI, United Arab Emirates — Israel launched a strike targeting Hamas' leadership in Qatar on Tuesday as they considered a U.S. proposal for a ceasefire in the Gaza Strip . The strike on the territory of a U.S. ally marked a stunning escalation and risked upending talks aimed at winding down the war and freeing hostages. Qatar, an energy-rich Gulf nation hosting thousands of American troops, has served as a key mediator between Israel and Hamas throughout the 22-month-old war and even before.