Video: This is why Germany and Japan are starting to arm up

More than seven decades after the start of World War II, Germany and Japan have begun to rearm.

These days, the countries are two of America's closest allies. But they can't singlehandedly project military power outside of their own borders.

In fact, the building of an offensive army is prohibited by the post-WWII constitutions of both Germany and Japan. And when the Cold War ended, it took a lot of the emphasis on building a strong military away from those vanquished nations.

Related: Japan's aircraft carrier comeback has been quiet and impressive

There just wasn't an enemy to fight that rivaled the threat posed by the Soviet Union.

Until now.

The rise of transnational terrorism sparked renewed efforts in developing Germany and Japan's defense capabilities. The two countries' defensive posture was designed around limited self-defense capabilities.

The U.S.-led invasion of Afghanistan in 2001 after the terror attacks on Sept. 11 was the first time NATO allies rallied and mobilized for mutual offensive action. Now, the threat of ISIS has made the need for an expanded military capacity even more pressing.

Learn more in the video below.