The trigger mechanism, which would split spending reductions equally between U.S. defense and domestic programs, is the product of months of unsuccessful negotiations among congressional Republicans, Democrats and the White House to strike a broader deal to rein in entitlement programs like Medicare and to overhaul the tax code.

While the cuts are supposed to be automatic, Congress can delay or override them if they prove too painful — defense spending would be reduced by 9.1 percent over a decade while non-defense programs would be cut 7.9 percent. That’s what lawmakers did with the 1985 Gramm-Rudman-Hollings Balanced Budget Act, the template for the trigger.