If Sen. Ted Cruz's defense plan was put into effect in fiscal 2017, he would be spending about $738 billion, or about $155 billion more than the Obama administration's ask for next year.

Not only is there little political will in Congress to approve that kind of spending, but it's also unclear what enemy Cruz is building up a military to fight, said Ben Friedman, an analyst with the Cato Institute.

The Texas senator and GOP presidential candidate unveiled his strategy to rebuild the military on Tuesday, including growing the Army to 525,000 active-duty soldiers, the Navy's fleet to at least 350 ships and the Air Force's inventory to 6,000 aircraft.

The Army's troop level in 2016 was about 475,000 soldiers, but is planning to draw down in fiscal 2017 to 460,000. The Air Force plans to have 5,472 airplanes, and the Navy is on track to grow to 287 ships in fiscal 2017.

To do this, he would commit to spending 4.1 percent of the gross domestic product on defense for the first two years of his administration, dropping to 4 percent in the remaining years. If that level of spending was implemented in fiscal 2017, it would come in at $738 billion. The fiscal 2017 Pentagon request would represent 3.35 percent of current GDP.

"Obviously Ted Cruz is not planning to raise taxes to do this, so it's hard to see where he's going to find the money," Friedman said.

The Obama administration has asked for $582.7 billion for fiscal 2017 for defense. Republicans in Congress have called on the Pentagon to spend more to cover continued operations in Afghanistan, the fight against the Islamic State and more reassurance for European allies. But even then, analysts predict Republicans will push to spend about $15 to $20 billion more.

In addition to not explaining where he would get the money to do this or how he would persuade lawmakers concerned about the deficit to boost defense spending this much, it's also unclear what enemy Cruz is building up to go after, Friedman said.

"I don't see what enemy it is that's going to justify that massive increase in force," Friedman said. "It's not projected against any particular enemy, it's just a big dollop for every category that they can think of."

Friedman predicted a large-scale conflict with China could warrant a larger Navy and Air Force, but that Cruz is not suggesting targeted investments. He also said a ground war against the Islamic State could require more troops, but that Cruz has so far said he would not put U.S. boots on the ground against the Islamic State.

For much of the Republican field, Friedman says he sees a gap in rhetoric from the Obama administration, but not a significant change in strategy.

"What they really want in terms of foreign policy is far less different from the current foreign polices than they'd ever admit," he said. "It's really doing the same thing harder."