KANSAS CITY, Mo. (Reuters) - President Bush has mismanaged the U.S. military and effectively instituted a "back-door draft," leaving forces ill-prepared to face new threats, Democratic challenger John Kerry charged on Thursday.

The decorated Vietnam War veteran said the United States had gone into Iraq with too few troops to prevent looting and crime and had neglected to secure almost a million tons of conventional weapons now being used against them.

"We failed to build alliances and squandered the opportunity to generate wider support inside Iraq, in the Arab world," Kerry said. "These mistakes have complicated our mission and complicated our objective."

The presumptive Democratic presidential nominee, in excerpts from an address on modernizing the U.S. military he will give at the Truman Presidential Library in Independence, Missouri, said change was essential now that the old Cold War enemy had been replaced by new threats from terrorism and weapons of mass destruction.

"To rise to the challenges we face, we must strengthen our military, including our Special Forces, improve our technology, and task our National Guard with Homeland Security," Kerry said.

The senator from Massachusetts, a 20-year veteran of the Foreign Relations Committee, will call for temporarily adding 40,000 troops to the 480,000-strong active-duty Army, more equipment and better training.

About 20,000 of those would be combat troops. The other 20,000 would be individuals with specialties in post-conflict skills, such as civil affairs and the military police in order to relieve the burden on the National Guard and Reserve.

Kerry blasted the Bush administration's order on Wednesday preventing thousands of soldiers designated for duty in Iraq or Afghanistan from leaving the military even when their volunteer service commitment expires.

He said the move to extend involuntarily the service of some soldiers underscored how overextended the U.S. military was as the Pentagon tries to maintain adequate troop levels in Iraq and Afghanistan.

"The administration's answer has been to put band-aids on the problem," he said. "They have effectively used a stop-loss policy as a back-door draft."

Extended tours of duty, delayed retirements and overuse of the Guard and Reserves meant the United States was in danger of creating "a hollow Army" that would not be ready for its 21st century mission, he said.

About 138,000 U.S. troops are in Iraq and some 13,000 in Afghanistan.

Even before Kerry spoke, Missouri's two Republican senators, Kit Bond and Jim Talent, accused him of political opportunism.

"Harry Truman loved to play poker, but John Kerry can't come to Independence and bluff his way through his record of indecisiveness and defense budget cuts," they said in a joint statement. "When Truman made a decision he stood by it. John Kerry sees major decisions as opportunities to advance his political career."

Bush and his Republican allies have tried to portray the Democrat as an equivocating liberal, soft on defense and weak on fighting terrorism, while casting the president as a strong war-time commander in chief.

"As president, I will use military force to protect our interests anywhere in the world, whenever necessary," Kerry vowed. "But strong leadership demands more than the willingness to use force. It means directing the use of the right tools at the right time for the right purpose and the right cause."