"The United States will lead, or the world will shift into neutral." Wise words from President Bush on May 20 to congressional Republicans. From the beginning, the president has made clear that we must lead and win the war on terror. To win the strategic war, we must of course win tactical battles. The central battle in the war on terror is Iraq. Unless we win that battle, we will see America itself, and the world, shift disastrously into neutral in the broader war.

In every war there are crucial turning moments, hard to foresee. They often occur in the midst of public despair about war prospects. Today there is considerable despair over the situation in Iraq. But despair existed in Britain and the United States after the fall of Singapore in World War II -- before the U.S. Navy's astonishing destruction of a Japanese carrier force in 1942 at Midway. In August 1864 there was a widespread belief in the North that the Civil War could not be won. President Abraham Lincoln believed that the war stalemate and the terrible casualties could lead to the election of his opponent, George McClellan, who might repudiate the Emancipation Proclamation and sue for peace on the basis of the status quo ante -- a free North, a slave South.

But Lincoln pressed forward. He argued that "no attempt at negotiation with the insurgent leader could result in any good. . . . He affords us no excuse to deceive ourselves. . . . Between him and us the issue is distinct, simple and inflexible. It is an issue which can only be tried by war, and decided by victory."

Then Atlanta fell to Union troops in the late summer of 1864. Lincoln was reelected, with 80 percent of the soldier vote. Shortly thereafter came the 13th Amendment, the abolition of slavery, the surrender of the Confederacy and the beginning of a long process of Reconstruction. Lincoln's war aims were ultimately realized.

What of the war aims of President Bush? He intends passage of sovereignty to an Iraqi government on June 30, and elections in January, followed by the establishment of a representative Iraqi government and the successful reconstruction of Iraqi society.

If a provisional Iraqi sovereign government is to operate effectively from July until the elected government takes power in January, adequate security is necessary. This requires striking a decisive military blow against the armed insurgencies that seek to prevent the Iraqi government from coming into existence. As was the case in 1864, the immediate task is therefore the destruction of the armies and militias of the insurgency -- not taking and holding territory, not winning the hearts and minds of Iraqis, not conciliating opponents and critics, not gaining the approval of other nations. All of these can follow after victory over the violent insurrection.

So any armed insurgency opposed to a peaceful transition in Iraq must be destroyed. Fallujah must be conquered and terrorists denied safe haven in Fallujah and other centers of insurrection. Moqtada Sadr's militia must be rendered powerless. This will have to be accomplished primarily by American and British military power -- however useful various political efforts can be, however useful Iraqi and coalition forces can be. Then a sovereign Iraq, with continued U.S. military and other assistance, will be able to move ahead with the task of political and economic reconstruction.

Such decisive military victories in Iraq would be respected by Sunnis, Shiites and Kurds alike. The new Iraqi government could then depend more confidently on Iraqi and American police and military power until it is ready to provide fully for its own police and military security.

Strategic success for the global war on terror depends on a decisive tactical victory over the armed insurgents of global terrorism in Iraq. Decisive military blows struck against violent opposition to the July passage of sovereignty and the January general election in Iraq would permit a supportable outcome at the polls in Iraq and the subsequent successful reconstruction of a democratic nation.

Meanwhile, as after William T. Sherman's victory in Atlanta, the reelection of the president at home would follow -- with a mandate to carry on, and to win, the global war against terror.

Lewis E. Lehrman, a former Republican candidate for governor of New York, is a partner in the investment firm L.E. Lehrman & Co. William Kristol is editor of the Weekly Standard.