President Barack Obama and Republican challenger Mitt Romney battled over Pentagon spending and Navy shipbuilding last night during the final presidential debate before the Nov. 6 election.

Obama mocked Romney, the former governor of Massachusetts, for emphasizing the naval fleet is smaller now than in 1917. Romney wants to increase weapons spending, notably calling for boosting Navy shipbuilding from nine to 15 ships per year–including increasing annual submarine production from two to three vessels.

Romney noted that the Navy in recent years has had a goal of boosting its fleet to 313 vessels, but now has 285 ships and could have fewer if so-called sequestration budget cuts to defense spending kick in in next year.

“That’s unacceptable to me,” Romney said at the event at Lynn University in Boca Raton, Fla. “I want to make sure that we have the ships that are required by our Navy,” he said, also lamenting that the Air Force is “older and smaller” than any time since its founding.

Romney and Obama both reiterated their opposition last night to the so-called sequestration cuts, the $1.2 trillion decade-long reduction in planned defense and non-defense spending that will start next January unless Congress and the White House can agree on an alternate plan. Romney has gone further and pledged to fight “$1 trillion” in defense cuts–the $500 billion from sequestration combined with $487 billion in decade-long reductions Obama supports and the Pentagon has budgeted.

“This in my view is the highest responsibility of the president of the United States, which is to maintain the safety of the American people, and I will not cut our military budget by $1 trillion,” Romney said. “That in my view is making our future less certain and less secure.”

Obama shot back that Romney “maybe hasn’t spent enough time looking at how our military works.”

“You mentioned the Navy, for example, and that we have fewer ships than we did in 1916,” Obama said. “Well, Governor, we also have fewer horses and bayonets, because the nature of our military’s changed. We have these things called aircraft carriers where planes land on them. We have these ships that go underwater, nuclear submarines.”

Obama said he and Pentagon leaders do not play “a game of Battleship where we’re counting ships.”

Romney has called for maintaining defense spending at 4 percent of the gross domestic product (GDP), leaving Obama supporters to question where he would find what they say is an additional $2 trillion needed to sustain that level of funding in future years. Romney told debate moderator Bob Schieffer, the CBS News journalist, that he would pay for more military spending by balancing the federal budget through steps including eliminating Obama’s health care program and reforming Medicaid.

Obama charged Romney’s federal budget proposal would not work, arguing his own spending plans would properly fund the military and also reduce the federal deficit.

The deficit, Obama said, “is a significant national-security concern, because we’ve got to make sure that our economy is strong at home, so that we can project military power overseas.”

Last night’s 90-minute debate was devoted to foreign policy–an issue mentioned only briefly during the two previous Obama-Romney showdowns. The candidates’ supporters were busy yesterday touting their military policy and budgeting stances.

Obama backer Lawrence Korb told reporters yesterday he sees Romney’s 4 percent of GDP goal as “irrelevant.”

That’s “because you’ve got to look at what you strategy is, you’ve got to look at that the threat is,” said Korb, a fellow at the left-leaning Center for American Progress Action Fund who served in President Ronald Reagan’s Pentagon.

Romney’s campaign previously said in an October 2011 whitepaper that he wanted to maintain a floor of 4 percent of GDP spending on “funds devoted to the fundamental military components of personnel, operations and maintenance, procurement, and research and development.” Yet, since then, some of his supporters have portrayed the 4 percent level as more of a rough goal, while others have described it as a target to be hit in future years.

Travis Sharp, a fellow the Center for a New American Security, calculated Romney’s proposal to maintain that defense spending at 4 percent of GDP would cost at least $2 trillion more over the next decade than would Obama’s defense-spending proposal.

Romney supporters took issue with Obama’s regular assertion that Romney wants $2 trillion in additional defense spending that the military didn’t request.

The pro-Romney Defending Defense Project–a joint effort of the Foreign Policy Initiative, American Enterprise Institute, and Heritage Foundation–criticized Obama yesterday for “repeatedly (attacking) Governor Romney’s plan to restore baseline military budgets to roughly 4 percent of domestic product as unnecessary spending that the Joint Chiefs of Staff don’t want.”

Defending Defense pointed in a statement to Obama’s call during a speech early last year to cut $400 billion in defense spending, highlighting that former Defense Secretary Robert Gates said he was not told in advance about those cuts.

Defending Defense quotes Gates as saying that suggestions “to cut defense by this or that large number have largely become exercises in simple math, divorced from serious considerations of capabilities, risk, and the level of resources needed to protect this country’s security and vital interests around the world.”

The group of conservative think tanks argues that Obama “has, conveniently, rewritten the standard of military strategy to conform to his defense-budget-cutting desires.”

Romney and his running mate Rep. Paul Ryan (R-Wis.), a congressman who chairs the House Budget Committee, have blamed Obama for the across-the-board sequestration budget cuts, which were created by the Budget Control Act of 2011. Journalist Bob Woodward says in a new book that the idea for the sequester–a package of politically unpopular cuts put in the law to try to compel Congress to agree on an alternate plan–came from the White House.

The Obama administration is opposed to sequestration, but also has threatened to veto legislation that tinkers with it without offering a viable alternate proposal to cut $1.2 trillion in federal spending. The president has called on congressional Democratic and Republican lawmakers to agree on a new wide-reaching plan that includes both spending cuts and new revenues.

Obama said last night that sequestration was proposed by Congress, not him. He predicted: “It will not happen.”