FORT BRAGG, N.C., June 28 (Reuters) - President George W. Bush will say in a nationally televised speech to Americans on Tuesday night that the Iraq war is vital to U.S. security and there will be "tough moments that test American resolve." "Amid all this violence, I know Americans ask the question: Is the sacrifice worth it? It is worth it, and it is vital to the future security of our country," Bush will say, according to speech excerpts released by the White House. The White House said Bush will acknowledge the tough fighting and suicide bombings in Iraq but...