The chairman of the Senate armed services committee, McCain will lead debate on the National Defense Authorization Act, one of the few remaining "must-pass" pieces of legislation that would authorize $700 billion in Pentagon spending and set a wide swath of military policy.
The measure is expected to be debated on the floor as early as this week, and it will set the pace as one of numerous national security issues Congress will tackle in the fall.
There have been major foreign policy developments around the globe since Congress was last in Washington, which is reflected in the double-barreled briefings scheduled
Wednesday on North Korea and Afghanistan.
The full House and Senate will receive separate classified, members-only briefings from Defense Secretary James Mattis, Secretary of State Rex Tillerson, Chairman of the Joint Chiefs Gen. Joe Dunford and Director of National Intelligence Dan Coats.
The briefings will be the first chance for skeptical lawmakers to question President Donald Trump's plan for Afghanistan, which he laid out in a speech last month.
Trump's call for more US troops with greater authority to attack the Taliban and other militant groups was praised by Republicans for committing US forces to winning the war in Afghanistan, but Democrats criticized the President for lacking a strategy and not providing details like the number of US additional troops that would deploy.
North Korea's latest nuclear test
on Sunday will refocus attention in Washington on the threat Pyongyang's nuclear program could pose to US allies -- not to mention the US itself.
Trump also stirred up concerns about conflict with North Korea in August when he warned of "fire and fury" against Pyongyang if it does not stop threatening the US. After the North's latest missile test that flew over Japan, US B-1B bombers and F-35B fighter jets joined with South Korean F-15 fighter jets in a joint flyover
of the Korean Peninsula as a show of force.
Trump has continued to tout potential military action over diplomacy, tweeting on Wednesday
that "talking is not the answer" for North Korea. His Cabinet, meanwhile, is emphasizing diplomacy. "We're never out of diplomatic options," Mattis said when asked about Trump's tweet.
But the following day, Mattis rejected the notion he was at odds with the commander in chief.
"There was nothing contradictory there," Mattis told reporters. "The President made very clear we are not talking to North Korea. ... there was no contradiction at all."
Some lawmakers have expressed concern that Trump could launch a preemptive military strike against North Korea, but there's not a lot Congress could do
to tie the President's hands short of explicitly barring military action, which is not likely to be considered.
Congress could, however, take steps toward drafting a war authorization for ISIS this fall. Mattis and Tillerson briefed the Senate Foreign Relations Committee on the issue before the August recess, and the panel's chairman Bob Corker has expressed openness to debating a draft authorization in committee that's authored by Democratic Sen. Tim Kaine of Virginia and Republican Sen. Jeff Flake of Arizona.
But the first item on the Senate's agenda will be the defense authorization bill, which could be the vehicle for a number of contentious debates over military policy.
Democrats are preparing
to try to reverse Trump's proposed ban on with an amendment to the defense bill, and Sen. Kirsten Gillibrand of New York has been sounding out moderate Republicans to find a provision that they could support.
The bill will attract scores of amendments, and party leaders will negotiate what issues will get votes on the Senate floor.
The bill would authorize a major boost in military spending, even more than Trump proposed in his budget, but securing the defense increase is no sure thing.
Congress is likely to pass a continuing resolution to prevent the government from shutting down on Oct. 1, which will keep funding at the same levels as the current year.
After that, there's still no clear path forward on a budget agreement to lift defense spending, and this fall the military is facing the most budget uncertainty
that it's seen in years.
In a Washington Post op-ed published Friday
, McCain urged Congress to return to regular order and find a bipartisan agreement on the defense budget, among other items.
"We all know spending levels for defense and other urgent priorities have been woefully inadequate for years. But we haven't found the will to work together to adjust them," McCain wrote. "A compromise that raises spending caps for both sides' priorities is better than the abject failure that has been our achievement to date."