Because the U.S. cast a vote about process, not principle. The Obama administration considers the Palestinian push for U.N. recognition an attempt to internationalize the Middle East peace process and isolate Israel. The United States has long argued that direct talks are the only solution, and fears that internationalization of the process will signal to Israel that Palestinian President Mahmoud Abbas isn't committed to bilateral negotiations. (Abbas has insisted that U.N. recognition is consistent with direct negotiations, and some influential Israelis concur.)
As for the 138 countries that voted "yes" on Thursday, their motivations are varied. Some argued that U.N. recognition of Palestine would pressure the Israelis to make concessions. Others share U.S. concerns that the resolution will hamper direct negotiations but still chose to make a symbolic vote in favor of Palestinian statehood. Voting against recognition of Palestine would have been a problem domestically for many leaders - especially if their rationale was based on a nuanced view of diplomatic process.
U.N. recognition of Palestine also has important legal ramifications. Palestine's new status will improve its chances of joining the International Criminal Court, where it could attempt to prosecute Israeli military and political officials for alleged war crimes committed in Gaza and the West Bank, including the construction of settlements in violation of the Fourth Geneva Convention. As it is, Israeli officials have refused to deplane in England after receiving reports that they could be arrested for war crimes. The United States, Britain and other countries pressed Abbas before Thursday's vote for assurances that Palestine wouldn't attempt to join the ICC, or at least wouldn't prosecute Israeli officials. Many took Abbas' refusal to provide such guarantees as an indication that Palestine intends to haul Israelis before the criminal court, ratcheting up international pressure against them.
The United States also objected to the timing of the U.N. resolution. President Barack Obama reportedly asked Abbas to delay the General Assembly vote, giving the United States time to reinvigorate direct negotiations between Israel and Palestine. When Abbas refused to honor Obama's request, he severely diminished the chances that the U.S. would vote "yes" on the resolution, or at least abstain. Concerns about timing are minor to most of the 138 countries that voted in favor of U.N. recognition of Palestine, because they aren't involved in negotiations.
(Explainer thanks Robert M. Danin of the Council on Foreign Relations.)
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