The legacy of the 2011 debt ceiling fight is the biggest issue Trump will face on day one

Five years ago, President Barack Obama signed into law what Todd Harrison, the director of defense budget analysis at the Center for Strategic and International Studies, says will be the biggest challenge for the incoming administration: the Budget Control Act of 2011.

"The Budget Control Act has dominated the budget debate in Washington for the past five years, and with a new administration coming into office in 2017 the Budget Control Act is likely to be on the top of their agenda," Harrison told Business Insider.

In short, after taking the highest office in the land, Donald Trump, the 45th President of the United States will have less than 300 days to negotiate a deal with Congress or see America's defense funds slashed.

"He has to cut a deal, and that is going to be a difficult task for a political outsider," Harrison said.

Here's what you need to know about the BCA and America's arms budget: