Washington ? Defense Secretary Robert Gates on Wednesday bluntly rejected suggestions that Afghanistan is a “war without end,” defending the U.S. military operation amid fresh congressional pressure on President Barack Obama to withdraw a significant number of U.S. troops beginning next month.

Gates acknowledged the war fatigue throughout the country and on Capitol Hill as the Afghan conflict nears the decade mark with lives lost and billions of dollars spent. But he argued that the United States and its NATO allies have made progress in building up Afghan security forces to prevent the al-Qaida terrorist network from a dangerous revival.

“I know people are frustrated. The country has been at war for 10 years. I know people are tired,” Gates told the Senate Appropriations defense subcommittee. “People also have to think in terms of stability and in terms of the potential of (al-Qaida) reconstitution. What’s the cost of failure?”

In the coming days, Obama will confer with Gates as well as senior military and administration officials and decide how many of the 100,000 U.S. troops in Afghanistan to withdraw in the initial round of troop reductions. Several lawmakers are clamoring for significant cuts, citing the killing of Osama bin Laden and CIA Director Leon Panetta’s assessment that less than 100 al-Qaida members remain in Afghanistan.

Twenty-seven senators, Democrats and Republicans, sent a letter to Obama on Wednesday pressing for a shift in Afghanistan strategy and major troop cuts.

“Given our successes, it is the right moment to initiate a sizable and sustained reduction in forces, with the goal of steadily redeploying all regular combat troops,” the senators wrote. “The costs of prolonging the war far outweigh the benefits. It is time for the United States to shift course in Afghanistan.”

Signing the letter was a fourth of the Senate, 25 Democrats and two freshman Republicans, Mike Lee of Utah and Rand Paul of Kentucky.

Gates insisted that the war is heading toward an end date of 2014 in a somewhat testy exchange with Sen. Patrick Leahy, D-Vt.

“This is not a war without end,” Gates said. “Troops will be coming down during that period. The costs of these wars is coming down dramatically. The cost of these wars will drop between fiscal 2011 and 2012 by $40 billion and between ’12 and ’13 probably by several tens of billions of dollars more. I think the prospects of having a more stable Afghanistan in terms of a country that can defend itself … I’m not talking about a Vermont democracy here.”

“Neither am I, Mr. Secretary, and you know that,” Leahy interrupted.

“I know,” Gates said, “but what I’m talking about is we are not in the business of nation building. What we are trying to do is build an Afghan national security forces to the point where they have the ability to defend that country and so that the Taliban and al-Qaida cannot reconstitute themselves in that country.”

Gates’ back-and-forth with Leady was an anomaly in what was likely his last congressional appearance after 4 1/2 years as Pentagon chief. Gates is retiring June 30 and several senators praised Gates.

“This nation is truly in your debt for turning the tide in Iraq and Afghanistan,” said Sen. Daniel Inouye, D-Hawaii.