ABOARD THE USS NIMITZ — Some 100 nautical miles northeast of Oahu in the Pacific Ocean, a fleet of U.S. Navy fighter jets slings from the deck of the USS Nimitz aircraft carrier, leaving thin trails of smoke on the tight runway.

The operation, part of maneuvers involving several thousand sailors as part of the world’s largest naval exercises in waters off Hawaii, was at the center of a growing controversy involving defense spending and foreign oil.

The dozens of air and sea vessels surrounding the Nimitz, including helicopters, fighter jets and destroyer ships, were running on a biofuel blend that can be substituted for traditional fuel without any engine modifications.

Navy officials say using the alternative fuel helps the military address weaknesses. Operations that use more than 50 million gallons of fuel each month rely on petroleum, making the U.S. military heavily dependent on foreign oil.

Market volatility causes Navy spending to swing by tens of millions of dollars each time the price of a barrel goes up or down $1.

“We’re not doing it to be faddish, we’re not doing it to be green,” Navy Secretary Ray Mabus said aboard the USS Nimitz on Wednesday. “We’re not doing it for any other reason except it takes care of a military vulnerability that we have.”

He added, “One of the things you better do as a military is take care of those vulnerabilities.”

But the plan to use a 50-50 blend of alternative and petroleum-based fuel has hit a snag — Congressional lawmakers who bristle at spending time and money chasing alternative energy at a time when defense spending is being cut and traditional oil is cheaper.

The Senate Armed Services Committee last month narrowly passed an amendment pushed by Republican Sens. John McCain of Arizona and Jim Inhofe of Oklahoma that would prohibit military spending on alternative fuels if their costs exceed the cost of traditional fossil fuels. The provision is now part of an overall defense budget bill making its way through Congress.

At the time, McCain said buying biofuel at $26 per gallon — the amount the Navy spent last year for 450,000 gallons of biofuels for this week’s demonstration — isn’t in line with priorities of pursuing energy technology that reduces fuel demand and saves lives.

Inhofe went further, saying the Pentagon “should not be wasting time perpetrating President Obama’s global warming fantasies or his ongoing war on affordable energy.”

The Navy, along with the U.S. Departments of Energy and Agriculture, is spending more than $500 million in pursuit of biofuels and other alternative energy sources like solar and geothermal. The nearly $12 million purchase on the fuel for the demonstration came at a time when the Navy was spending just below $4 per gallon for traditional marine and jet fuel, according to Navy energy officials. The price has dipped dramatically since then, but is expected to rise to about $3.60 by the time the next fiscal year begins.

Mabus said the price of biofuels and other alternative sources will go down dramatically if the military makes massive purchases. Private industries, including the commercial airline industry, are interested and will join to help lower market prices, he said.

“I’m confident that as we get the story of what the Navy is doing completely out there, as we talk to members of Congress as we have been as we’ve been engaging them, that we will reach a common ground here in terms of making sure that America moves toward energy independence, making sure that America moves toward energy security,” Mabus said.

Mabus said the Navy is still pressing ahead with its goal of getting half its fuel from alternative sources by 2020.

Two senators on the Armed Services Committee, Democrat Jeanne Shaheen of New Hampshire and Republican Susan Collins of Maine, have vowed to try to change the defense bill once it reaches the Senate floor, writing in a Politico column last week that the biofuels investments will result in long-term savings and are part of an “all-of-the-above” approach. Collins was absent during the committee vote, where the biofuels amendment passed 13-12.