President Bush told thousands of National Guard members and their families on Wednesday that an immediate withdrawal of American troops from Iraq would only embolden terrorists and make America and its allies more vulnerable to attack.

Defending his administration’s military stance for the third day in a row, he presented another tough, if implicit, rebuttal to war critics like Cindy Sheehan, the mother of an American soldier killed in Iraq who has generated a monthlong protest outside his Texas ranch. Mr. Bush said, “As long as I’m the president, we will stay, we will fight and we will win the war on terror.”