The war in Afghanistan may be winding down. But the Pentagon's chief of irregular warfare still sees a war against al-Qaida that will last decades, all over the world – a prospect that prompted astonishment and constitutional debate in the Senate.

Asked at a Senate hearing today how long the war on terrorism will last, Michael Sheehan, the assistant secretary of defense for special operations and low-intensity conflict, answered, "At least 10 to 20 years."

It was just two months ago that the top U.S. intelligence official testified that al-Qaida had been battered by the U.S. into a state of disarray. A year ago, the current CIA director, John Brennan, said that "For the first time since this fight began, we can look ahead and envision a world in which the al Qaeda core is simply no longer relevant." Just this week, the commander of the Joint Special Operations Command, Army Lt. Gen. Joseph Votel, told a Florida conference that he was looking at missions beyond the counterterrorism manhunt.

Yet a spokeswoman, Army Col. Anne Edgecomb, clarified that Sheehan meant the conflict is likely to last 10 to 20 more years from today – atop the 12 years that the conflict has already lasted. Welcome to America's Thirty Years War.

There is no geographic limit to that war, Sheehan and others testified, thanks to the seminal law authorizing it in the days after 9/11, known as the Authorization to Use Military Force. Thanks to that relatively terse authorization, U.S. counterterrorism stretches "from Boston to the FATA," Sheehan said, using the acronym for Pakistan's tribal areas. Sheehan told the Senate Armed Services Committee that he believed "associated forces" of al-Qaida can be targeted anywhere around the world, including inside Syria, where the rebel Nusra Front recently allied itself with al-Qaida's Iraq affiliate, or even what Sen. Lindsey Graham (R-SC) called "boots on the ground in Congo."

Asked if individuals who were not born on 9/11 but join al-Qaida are legal targets for the U.S. military, acting Pentagon chief lawyer Robert Taylor answered, "As long as they become an associated force under the legal standard that was set out."

That extensive authority, particularly inside the United States prompted a highly publicized protest on the Senate floor by Sen. Rand Paul (R-Ky.) in March. Members of the panel expressed shock that Sheehan and Taylor envisioned such a broad, long war.

"You guys have essentially rewritten the Constitution here," said Sen. Angus King (I-Maine). King pointed out that the 2001 Authorization to Use Military Force against al-Qaida, or AUMF, was specifically bounded to avenging the 9/11 attacks, and does not contain the words "associated forces" repeatedly invoked by Taylor, the top Pentagon lawyer.

"You guys have invented this term, associated forces, that's nowhere in this document," King said. "It's the justification for everything, and it renders the war powers of Congress null and void." Sen. John McCain (R-Ariz.), expressed incredulity over the Pentagon's broad interpretation of the AUMF. "None of us" who voted for the law in 2001 "could have envisioned [granting] authority [to strike] in Yemen and Somalia," McCain said.

The panel's chairman, Sen. Carl Levin (D-Mich.), jumped to the Pentagon's defense, saying that "any co-belligerent" was an appropriate target under established principles of international law.

While the Pentagon takes an expansive view of the powers Congress granted it under the AUMF, it has resisted for years efforts by Congress to revise the document. Taylor's predecessor, Jeh Johnson, rejected the House Armed Services Committee's 2011 push to vote on an updated AUMF, although more than half of currently-serving legislators were not serving when the 2001 law passed. It's unclear how much appetite Congress has for revising the AUMF; nor whether such a revision would restrict U.S. counterterrorism efforts or expand them even further, to target groups or individuals unrelated to al-Qaida.

The only circumstances under which the administration might need new authorities for counterterrorism, Sheehan said, would be if a new terrorist group unaffiliated with al-Qaida arose to threaten the United States. Sheehan said that he saw that as a mere hypothetical. "Existing authorities are adequate for this armed conflict," Taylor reaffirmed today.

Like Johnson did last year, the Pentagon officials also said that they saw "a tipping point" whereby al-Qaida is defeated. When that happens, the officials testified, the armed conflict against al-Qaida will end. ("That day, unfortunately, is a long way off," Sheehan said.) Taylor added that such a conclusion to the conflict won't necessarily nullify the AUMF. "If the president were to issue a declaration that the conflict against al-Qaida has been concluded," he said, "I think that would constitute an end," as would a repeal by Congress.

"I've never seen such an accomplished group of people give such muddled answers," testified Rosa Brooks, a former Pentagon policy official.