Deterrence relying on the nuclear triad of bombers and land- and sea-based missiles is not enough to dissuade potential attackers in the future, said the vice chairman of the Joint Chiefs of Staff. General James Cartwright argued that spec ops troops, conventional forces, and the ability to rapidly deploy both must be part of a new construct.

This is in contrast to the immediate global strike capability that the U.S. currently has: nuclear warheads on intercontinental ballistic missiles, he said.

"We can't just stay in nuclear and say that's deterrence," Marine Gen. James Cartwright told reporters today. "Because there's so much in the spectrum of conflict that nuclear weapons is just not relevant. The question is, can you portray those forces in such a way that's convincing that they can do whatever it is we say they can do … and get them to wherever they need to get in a reasonable period of time."

Cartwright said the U.S. has to redefine deterrence going forward.

With some countries, he said, the historic nuclear triad – which essentially carries with it the guarantee of mutually assured destruction, or MAD – is enough to deter an attack on the U.S.

But MAD is not a one-size-fits-all option, according to Cartwright.

"We need to engender discussion about what deterrence looks like when we get out to 2020/2030," he said. "If you want deterrence for the 21st century, it ought to be the whole of government, not just kinetic power. You may actually stay with MAD with one country, but not the other because that doesn't compel us."

There are also threats from violent extremist groups, non-state actors who are also attempting to get hold of weapons of mass destruction, he said.

"It's equally threatening so we have to start to think about this a little more holistically," he said.

That approach also means being able to communicate to with state and non-state actors alike that the U.S. has the ability to take out things that they value or hold dear if they attack.

You have to be able to convince them "that if you want to take this to a fight, you're not going to win," he said.