 -- When he took the oath of office on a drizzly January day, President Trump swore to put his “America First” foreign policy into action, laying out a path to upend the status quo and, in his view, bring America back to primacy.

“It is the right of all nations to put their own interests first. We do not seek to impose our way of life on anyone but rather to let it shine as an example,” he said, promising to “reinforce old alliances and form new ones -- and unite the civilized world against radical Islamic terrorism.”

Since then, his first 100 days have been filled with surprises as he and his team steer through early challenges around the world as they learn to steady the ship -- and there are plenty more on the horizon for an administration that’s still working to define the Trump doctrine.

NORTH KOREA AND A NUCLEAR BOMB

The administration continues to focus on the urgent threat from North Korea, where strongman Kim Jong Un is testing ballistic missiles at a high tempo and reportedly stands on the brink of another nuclear test.

The regime is aiming for an intercontinental ballistic missile tipped with a nuclear warhead, capable of reaching the mainland United States, and with every test, they learn something and inch that much closer.

The Trump administration has focused its efforts on convincing China to do more, with Secretary of State Rex Tillerson chairing a special session of the United nations Security Council Friday to pressure them even more.

North Korea’s neighbor and largest trading partner, China has been called upon by the international community to help rein in the Kim regime's nuclear and missile programs, but the Chinese deny that they have that influence. What’s more, they fear too much pressure will crumble the regime, unleashing a wave of refugees over their border.

Complicating the threat even more is the South Korean presidential election on May 9, where the liberal party is expected to win back power. If they do, they would likely take a softer stance with the North and seek negotiations, standing in contrast to Trump’s harsh tone. The differences with Trump, including over the THAAD missile system that the U.S. just installed in South Korea and that Trump wants South Korea to pay for, could rupture this critical alliance.

ISIS -- AND WHAT COMES NEXT

While nuclear-armed North Korea may seem like a looming danger, there is an immediate threat to American lives -- those American troops on the front lines against ISIS. That risk was made very real again this past weekend, when two soldiers were killed in Afghanistan and one in Iraq.

In total, the U.S. has around 900 troops fighting ISIS in Syria and about 6,000 in Iraq, with 8,500 more in Afghanistan, where they’re fighting both the Taliban and ISIS while training Afghan security forces.

Despite some setbacks in Afghanistan, the fight against ISIS is progressing steadily in Iraq and Syria. As U.S.-backed forces advance on ISIS’s last strongholds in Syria and Iraq with American artillery and air support, the focus will turn to what comes next -- and battlefield victories are not the end. Once terrain is taken back, the U.S. and its coalition partners have to focus on keeping ISIS out and stabilizing the area to prevent any new terror group from forming.

Security officials are also concerned that the end of ISIS’s so-called caliphate would mean a flood of foreign fighters back to their homelands, creating a new threat in the West and leading to more one-off attacks like the one earlier this month in Paris.

THE RELATIONSHIP WITH CHINA

Now in office, Trump’s repeated attacks on China on the campaign trail are gone, as the new administration has sought to work together to solve some pressing issues. Where once there was talk of tariffs or closer ties to Taiwan, there is now nothing but praise for Chinese president Xi Jinping.

“He’s doing an amazing job as a leader,” Trump told Reuters Friday. “I have established a very good personal relationship with President Xi. I really feel that he is doing everything in his power to help us with a big situation.”

But so far, China has not taken forceful steps to deal with that big situation, North Korea -- and if they don’t act, at some point it could hurt relations with Trump.

Beyond that, there are still a host of differences that Trump built his campaign highlighting, especially trade. Resolving them will be a delicate dance for the world’s two largest economies, but returning to that heated campaign rhetoric could be an alluring relief valve for a president under intense scrutiny and unpopular back home.

RUSSIAN ROULETTE

Despite the swirling questions and ongoing investigations into his and his team’s ties to Russia, Trump still says he wants to work with Moscow.

But the administration has not lifted any sanctions on Russia, hit its ally Bashar al-Assad with strikes, and has had some harsh words for its human rights record and continued support of the Syrian regime. The White House may be taking a harder line or trying an “Art of the Deal” tactic to negotiate, but a moment of crisis could quickly come to test them.

That could be new Russian aggression in eastern Europe or buzzing American ships and jets around the world. The eyes of NATO would turn to Washington to see how Trump would react and whether he’d stand by the western alliance, as he now says he would.

There’s also the danger of an incident in Syria erupting into all out war: American soldiers accidently bombed by Russian jets targeting Assad’s enemies -- or the brutal dictator recrossing America’s red line with more chemical weapons attacks. Trump shocked the world once with his punitive strike on Assad; it’s uncertain how he would respond in the future.

NORTH AMERICAN NEIGHBORS

One surprising challenge for the Trump White House may be closer to home.

Every time Trump bashed China on the campaign trail, he also attacked Mexico, blaming Mexico for a trade imbalance, the loss of American manufacturing jobs, and a loose border that allowed drugs, crime, and undocumented immigrants to pass through. The rhetoric incensed the Mexican public -- and that ill will has carried over into his time in office, including the idea that Mexico will pay for a border wall.

Depending on Trump’s next move, Mexico could retaliate -- if Trump slaps a tariff on Mexican goods to the U.S.; if he tries to tax remittances from the U.S. to relatives in Mexico; and if U.S. Immigration and Customs Enforcement starts to deport undocumented immigrants to Mexico, regardless of their country of origin.

Last week, Trump also started picking a fight with Canada over trade -- leading to at least two urgent phone calls between him and Canadian prime minister Justin Trudeau. The two sides, along with Mexican president Enrique Pena Nieto, agreed to renegotiate NAFTA later this year. But if Trump doesn’t like the deal he gets, relations in North America could get ugly.