Don't blame the United States. Sharon recently said that Colin Powell had just told him in a phone conversation that the U.S. understands and fully supports Israel's need to defend itself. Sharon's problem is that his support among the Israeli public is slipping dramatically. The Likud wants him to drive the Palestinians back across the Jordan. But if he does Labor will pull out of his coaltion, the government will fall and he'll have to call for new elections, which, if the polls are any indication, he'll easily lose to Netanyahu. So to stay in power, Sharon does enough to satisfy the right wingers but not so much as to offend the left. It's not much of a policy and it prevents him from taking any bold action to settle this once and for all (including taking risks for peace).
Sharon's only hope (I've said this before) is for the U.S. to attack Iraq. Saddam will retaliate by firing SCUDS at Israel. The Palestinians will go wild with joy and stage massive attacks on Israelis. Sharon will send in the IDF, ostensibly to stop the violence but actually to drive the Palestinians out of Gaza and the West Bank at a time when the eyes of the rest of the world are on the U.S. war with Iraq.
But Sharon can't do anything till the U.S. attacks Iraq. That's why he's so frustrated. He won't have the moral capital to ethnically cleanse the Palestinians till the U.S. attacks Iraq and Saddam in turn attacks Israel. In the meantime, he just has to sit there and take it.