WASHINGTON – The Pentagon ordered 90-day extensions Wednesday for all active-duty Army troops in Iraq and Afghanistan, stretching their overseas tours from 12 months to 15 months in a move that will exert new strain on a struggling military but allow the Bush administration to continue its troop buildup in Baghdad well into next year.

Defense Secretary Robert M. Gates’ announcement came amid expectations that the Pentagon was about to order longer tours for some units – but the new policy is a far more sweeping and drastic step, stretching deployments for more than 100,000 members of the Army.

“I realize this decision will ask a lot of our Army troops and their families,” Gates said, while adding that it would ensure that the administration would not be forced to withdraw forces before it was ready. “This approach also upholds our commitment to decide when to begin any drawdown of U.S. forces in Iraq solely based on conditions on the ground.”

The extension order also came at a crucial time in the war and the political debate surrounding it as congressional Democrats push for troop withdrawals.

It marked the second time in four months that the administration responded to pressure for troop withdrawals by taking a dramatic step to expand the U.S. involvement in Iraq. The Iraq Study Group recommended troop withdrawals in December, just weeks before President Bush announced the current buildup.

Gates said the extensions were not a signal that he had decided to continue the troop buildup. But military experts said that by extending all of the active duty brigades, the administration would be able to continue the surge into 2008.

“It was always envisioned that the only way you could do it (the troop increase) was to extend tours of duty; that was known right from the outset,” said Jack Keane, a retired Army general and one of the architects of the current strategy, who recommended across-the-board extensions last December.

Gates’ decision will immediately affect approximately 18,000 soldiers in Afghanistan, 79,000 soldiers in Iraq, and another 7,000 in Kuwait, according to Army officials. The first combat units to be affected in Iraq will be the units that were due to come home this summer.

The extensions do not affect the Marine Corps, whose members currently serve for seven months in Iraq before returning home for six months, or the National Guard.

The Minnesota National Guard’s yearlong tour was extended in January, when Bush announced the troop increase. But the Pentagon officials since have promised they would only mobilize those soldiers for one year at a time, including training, which means their Iraq tours are likely to be about 10 months.

William Nash, a retired Army major general now at the Council of Foreign Relations, is among experts who see a likely connection between the extensions and the flexibility to continue the troop buildup.

“The fact is we are going to keep doing what we are doing,” Nash said. “Absolutely, it will go into next year. That is why they went to 15 months.”

The extensions also may signal that the administration believes that the initial buildup of forces is having a positive effect in Baghdad and in Anbar province, said Andrew F. Krepinevich Jr., a former Army major and an expert on counterinsurgency strategy who heads the Center on Strategic and Budgetary Assessments. “We are reinforcing success; some things are breaking right for us,” he said “Maybe Secretary Gates wants to generate real success so Congress will get off his back.”

Toni Johnson, 24, thought she was prepared for life as a military wife when she married her high school sweetheart, Jeff, an Army mechanic based at Fort Hood, Texas. But an extended war, and now news of a 15-month deployment, is testing her patience, she said.

“That’s three extra months without my husband, three extra months of our kids missing their father,” said Johnson, a Republican who voted for Bush. “It will definitely be a hardship.”

The couple have three children, ages 3, 4, and 9. “It’s hardest on the younger ones because they don’t understand why Daddy is gone,” Johnson said. “I underestimated how hard it is to be a military wife.”