The second paragraph of a NYT news story on the cuts in military spending implied by the budget agreement from last summer told readers:
"On Jan. 2, national security is set to receive a heavy blow if Congress fails to intervene. That is when a 10-year, $600 billion, across-the-board spending cut is to hit the Pentagon, equal to roughly 8 percent of its current budget."
While the size of these cuts is not in dispute, it is far from clear that they would constitute a "heavy blow" to national security. Even if the cuts were fully implemented, the United States would still be spending considerably more as a share of GDP on the military in 2022 than it did in 2000, before the September 11th attacks.
It is also worth noting that there is nothing that happens on January 2 that locks in the scheduled cuts for a decade. If Congress determined in 2017 or 2018, or even some time earlier, that our defense was in jeopardy then it would presumably vote to increase spending to meet the perceived threat.