There's still no end in sight to America's longest war in Afghanistan. North Korea hasn't given up its nuclear weapons.

Add to that simmering tensions with Iran, fallout from Trump's decision to pull troops from Syria, ongoing unease with Russia and Turkey, and erratic ties with European and other longtime Western allies.

Trump is not popular overseas, and being an impeached president who must simultaneously run for re-election could reduce the time, focus and political clout needed to resolve complex global issues like North Korea's nuclear provocations.

Some foreign powers could decide to just hold off on finalising any deals until they know whether Trump will be re-elected. Trump himself has acknowledged the challenge in his December 26 tweet:

"Despite all of the great success that our Country has had over the last 3 years, it makes it much more difficult to deal with foreign leaders (and others) when I am having to constantly defend myself against the Do Nothing Democrats & their bogus Impeachment Scam. Bad for USA!"

At the same time, there is widespread expectation that Trump never will be convicted by the Republican-controlled Senate, so 2020 could well bring more of the same from the president on foreign policy.

For Trump, 2019 was a year of two steps forward, one step back - sometimes vice versa - on international challenges. Despite claiming that "I know deals, I think, better than anybody knows deals", he's still trying to close a bunch.

Trump scored high marks for the US military raid in Syria that killed the leader of the Islamic State, but US military leaders worry about a resurgence.

He is credited with coaxing NATO allies to commit to spend billions more on defence, but along the way has strained important relationships.

His agreement on a "Phase 1" trade deal with China has reduced tensions in their ongoing trade war. But the deal largely puts off for later complex issues surrounding US assertions that China is cheating to gain supremacy on technology and China's accusation that Washington is trying to restrain Beijing's ascent as a world power.

As 2020 begins, Trump faces three top foreign policy challenges.

US-NORTH KOREA NUCLEAR TALKS LOSE TRACTION

The US is watching North Korea closely for signs of a possible missile launch or nuclear test. In recent months, North Korea has conducted a slew of short-range missile launches and other weapons tests.

Pyongyang had threatened to spring a "Christmas surprise" if the US failed to meet Kim Jong-un's year-end deadline for concessions to revive stalled nuclear talks. Any test flight of an intercontinental ballistic missile or substantial nuclear test would further derail the diplomatic negotiations Trump opened with Kim in 2018.

Washington didn't accept Kim's end-of-year ultimatum but Stephen Biegun, the top US envoy to North Korea, said the window for talks with the US remains open.

US-IRAN TENSION ESCALATING

Tensions with Iran have been rising ever since Trump last year withdrew the US from the 2015 nuclear deal that Tehran had signed with the US and five other nations. Trump said the deal was one-sided and gave Iran sanctions relief for rolling back, but not permanently dismantling, its nuclear program.

After pulling out of the deal, Trump began a "maximum pressure" campaign, reinstating sanctions and adding more that have crippled Iran's economy. His aim is to force Iran to renegotiate a deal more favourable to the US and other nations that are still in the agreement.

In response, Iran has continued its efforts to destabilise the region, attacking targets in Saudi Arabia, interrupting commercial shipping through the critical Strait of Hormuz, shooting down an unmanned US aircraft and financing militant proxy groups.

The White House says its pressure campaign is working. The Iranian economy is collapsing, inflation is high. And crushing US sanctions blocking Iran from selling its crude oil abroad have helped fuel nationwide protests.

Earlier this month, there was a rare diplomatic breakthrough when a Chinese-American Princeton scholar, Xiyue Wang, who has held in Iran for three years, was freed in exchange for a detained Iranian scientist in the US

Trump said the prisoner exchange could be a "precursor as to what can be done."

Iran says other prisoner swaps can be arranged, but there will be no other negotiations between Tehran and the Trump administration.

AFGHANISTAN

When Trump made his first visit to Afghanistan on Thanksgiving Day, he announced that negotiations with the Taliban, which had fallen apart in September, were back on track. He claimed the militant group wanted to find a political resolution to the war, now more than 18 years old.

Less than two weeks later, talks were back on pause after an attack outside Bagram killed two Afghans and wounded 70 others, including members of the US-led coalition force.

It's no secret that Trump wants US engagement in Afghanistan to end, but critics worry that this will lead him to make too many concessions to the Taliban.

This time around, the US is seeking a reduction in violence with the end goal of getting the Taliban to agree to a permanent ceasefire and start all-Afghan talks to find a peaceful future for the country.

Senator Lindsey Graham, who visited Kabul this month, said Trump could reduce troop numbers to 8,600, down from the current estimate of 12,000.

The Taliban have said any peace agreement must include getting all American troops out of the country, where more than 2,400 American service members have been killed.

Australian Associated Press