Meeting at the White House, Friday, President Trump had kind words for the newly elected President of South Korea.

Watching the two leaders together in the Rose Garden was interesting. It's clear that they get along.

I say interesting, because Moon Jae-in is an unlikely Trump ally. A former human rights lawyer, Moon adopts the polar opposite of Trump's bombastic approach to governing. Yet President Moon is also a clever politician. He knows instinctively, what President Emmanuel Macron of France had to learn. Namely, that praising Trump goes down better than teasing him. Moon actively showered his host with praise.

Still, Trump shouldn't give Moon a free ride. Because on one issue in particular, the South Korean government must do more to support shared U.S.-Korean interests.

Defense spending.

While South Korea spends a higher percentage of its gross domestic product on defense than many U.S. allies in NATO, its 2.5 percent expenditure is inadequate. Facing the North Korean threat, and recognizing the vast commitment of U.S. personnel and equipment to its defense, South Korea should be spending 4 to 5 percent of GDP on defense. And that's at a minimum.

Some disagree. Speaking to reporters prior to Trump's joint statement with Moon on Friday morning, an unnamed U.S. official suggested that when it comes to defense spending, "we shouldn't view South Korea as somehow laggard on that front." This official, like others, believes that because South Korea is spending more than wealthier allies like Germany (1.3 percent), it deserves a break.

But this unnamed official is wrong. Germany doesn't share a border with North Korea. As the threat posed by Kim Jong Un continues to grow, the U.S. Military will strengthen its posture in South Korea. It will be expensive.

Were South Korea to raise its defense budget by around 50 percent over five years (to 4 percent of gdp), it would help match the U.S. commitment. And those new funds could be pledged to critical needs. For one, boosting South Korea's close air support capability. In the event of a conflict with North Korea, the order of battle will involve the U.S. Air Force taking the lead in destroying the North's air force and establish air superiority.

At the same time, however, U.S. and South Korean troops on the ground will need the support of slow, accurate aircraft that can identify and destroy North Korean ground units. If they don't have sufficient numbers of those aircraft at their side, they will die in greater numbers.

Unfortunately, U.S. commanders are stretched by an insufficiency of pilots and a multitude of global taskings. They need help. And South Korea is well-placed to fill the gap. Moreover, it has the interest in doing so. After all, if North Korean forces are able to punch through U.S.-South Korea lines, Seoul will fall.

Ultimately, any alliance must be based on shared commitment in both mission and means. The U.S. is rightly committed to South Korea's defense and is putting the resources in play to assure that mission. But while South Korea is equally driven by its own survival, it can and should do more to support the mission.