President Barack Obama on Tuesday night will announce that he is sending an additional 30,000 troops to Afghanistan with the aim of beginning to withdraw U.S. troops from the country in July 2011, according to senior administration officials.

The officials, who spoke to reporters via teleconference, emphasized that while Obama will pinpoint the beginning of the drawdown, he will not indicate how long the withdrawal will take, saying this will depend on conditions on the ground.

Obama will assert the commitment is not “open ended,— but he will not put a timeline on ending U.S. participation in the conflict, the officials said.

“If the Taliban thinks they can wait us out, then they’re misjudging the president’s approach,— one of the senior administration officials said. He indicated the setting of a date to begin withdrawal is designed to put pressure on the process to move forward, not to suggest to the enemy when the United States will abandon the fight.

The officials indicated that the role of the additional troops — who will be supplemented by troops supplied by U.S. allies — will be to weaken the Taliban in order to allow Afghanistan’s military and police to finish them off. The new troops will also be responsible for training Afghan military and police.

The United States will also seek to expedite rebuilding in the country. The top Afghanistan development priority will be agriculture, one official said. This appeared to suggest a stepped-up effort to combat widespread growing of poppies by Afghan farmers.

The officials said the administration’s goal is to get the troops into Afghanistan by next summer.