Secretary of State Warren Christopher and Defense Secretary William Perry today tried, without apparent success, to persuade the Senate Armed Services Committee to back the Clinton Administration plan to send more than 20,000 U.S. peacekeepers to Bosnia. Only two of the 21 members, Democrats John Glenn of Ohio and Charles Robb of Virginia, said they supported the idea. President Clinton has already promised to supply a third of the force thought necessary to maintain order in the Balkans after peace becomes official. But Defense correspondent Mark Thompson reports that the planned mission remains mired in arguments over its length and terms. "At a breakfast this morning, General George Joulwan, the four-star the overall commander of NATO forces in Europe, argued that you need to have a definite timetable for when the mission will be done, in this case 12 months and then the troops will pull out," Thompson says. "But Perry has said that the U.S. would stay if the situation was collapsing."