The extra dollars for the military would help fund the US wars in Afghanistan, Iraq and Syria, as well as replenish equipment and pay for training and maintenance.
Some of the extra funding would come with strings attached: The bill would fence off $2.5 billion of the $15 billion until the Trump administration sends a plan to Congress explaining the US strategy to defeat the Islamic State of Iraq and Syria.
Trump asked the Pentagon to send him a plan to defeat ISIS within 30 days, but the plan has not been released publicly.
After the Trump administration asked for the $30 billion defense funding supplemental last month, Democrats grumbled that the request included $5 billion specifically for the war against ISIS without explaining how the campaign would be accelerated.
"What are we getting ready for? What are the priorities?" Illinois Sen. Dick Durbin, the top Democrat on the Senate Defense Appropriations Subcommittee, said at a hearing with Defense Secretary James Mattis and Joint Chiefs of Staff Chairman Gen. Joseph Dunford last month. "That's not a hard question — it's an important question if we were being asked to appropriate this money."
The additional Pentagon funding adds to the $578 billion in defense spending included in the omnibus bill, which will get a vote this week ahead of a Friday deadline to prevent a government shutdown.
On the policy side, the spending deal would provide funding for the military to tackle its recent nude photo scandal, with $18 million for "consulting services to include assistance to the commandant of the Marine Corps to address the Marines United scandal."
The agreement also would authorize 2,500 new special immigrant visas for Afghans who have aided the US in Afghanistan.
The additional supplemental funding would replace aircraft lost in combat, including a V-22 Osprey tilt-rotor aircraft, a UH-60 Blackhawk helicopter, two AH-63 Apache helicopters and eight MQ-1 drones.
It includes $85 million to replenish the Navy's Tomahawk stockpile, which were the missiles used by the US to strike Syrian government targets last month in response to a chemical weapons attack. And it provides $151 million for the Terminal High Altitude Area Defense, or THAAD, the missile defense system that's being deployed to South Korea.
In addition, there's also about $6 billion from the $30 billion supplemental request that the House had already included in its defense spending bill last month, according to congressional aides.
To get the extra funding, Congress labeled the extra $15 billion as war funding, outside of the base budget. That may seem to be an archaic difference, but it makes a big difference: War dollars are not subject to the budget caps that limit the total amount of defense and non-defense spending.
The spending deal also did not include $18 billion in cuts to federal agencies that the Trump administration had sought to offset the extra funding for the military.
The deal to give the Pentagon an extra $15 billion would provide the military with funding stability through September, but next year's budget request is shaping up to be an even bigger fight.
The Trump administration plans to ask for an extra $54 billion in military spending above the budget caps while cutting the same amount from domestic spending.
The cuts are non-starters to Democrats, and even some Republicans have criticized the potential deep cuts to the State Department.
At the same time, Republican defense hawks want more funding to rebuild the military, and Mattis has privately told lawmakers
that the Trump administration's planned defense boost still isn't enough to rebuild the military as Trump has vowed to do.