Yesterday, President Obama vetoed the FY2016 Defense Authorization bill (H.R. 1735), which passed congress early this month. The president has threatened to veto defense authorization bills throughout his presidency, but this is the first time he has actually taken the veto pen to the defense bill.

Although the president identified opposition to provisions in the bill about detainees at Guantanamo and Congress’ failure to approve requested defense reforms, the main reason for the veto is White House opposition to $38 billion of base budget requirements that the bill includes in funding for Overseas Contingency Operations (OCO). OCO funding is considered emergency funding not constrained by the budget caps.

The president and most Democrats oppose this approach because they believe it circumvents the budget act to increase defense spending and could lead to significant cuts to nondefense programs.

In his veto statement, the president called this use of OCO to fund defense base budget requirements “an irresponsible budget gimmick” and charged it “does not provide the stable, multi-year budget upon which sound defense planning depends.” This sentiment has been echoed by Secretary of Defense Ash Carter.

Congressional republicans called the president’s veto irresponsible. House Armed Services Committee chair Rep. Mac Thornberry (R-TX) said the veto was unprecedented and reckless and that the House would move to override.

It is uncertain, but unlikely that Republicans will be able to garner enough votes to override the veto. While some Democrats voted for the final bill in both the House and Senate, Democrat leaders believe they have enough votes to thwart any effort to override.

Both the House and Senate need a two-thirds vote to override the president’s veto. Rep. Thornberry announced the House will hold an override vote in early November. If the override fails in the House, a Senate vote cannot hold an override vote.

If Congress does not override the veto, the bill will go back to the committees to try to work out a bill that the president will sign.

This is President Obama’s fifth veto, a historically small number of vetoes for a president. Congress did not override any of the other four Obama vetoes.