Bush Urges World to Unite With Iraq UNITED NATIONS - President Bush, defending his decision to invade Iraq, urged a vast assembly of world leaders Tuesday to stand united with the country's struggling government and said the proper response to spreading violence "is not to retreat, it is to prevail."   The country's prime minister, Ayad Allawi, offered an upbeat assessment after Bush's speech to the General Assembly of the United Nations, saying, "We are winning, we are making progress in Iraq, we are defeating terrorists," even as insurgents claimed they had killed a second American hostage in two days.    Of the brutal slayings, Bush said, "We will not allow these thugs and terrorists to decide your fate and to decide our fate."   Yet in a sign of continuing world unease with the situation, U.N...