During President Donald Trump's trip to Asia, he expressed interest in wading into the dispute over the South China Sea.
He told leaders on Sunday: "I know you've had a dispute for quite a while with China. If I can help in any way, I'm a very good mediator and a very good arbitrator."
The issue came to the fore while Trump was in Vietnam. Along with Vietnam, Taiwan, Malaysia, Brunei, the Philippines and China all claim parts of the water and land in the South China Sea. Per international law, most of them only claim water up to 200 miles off their coasts — that's called an exclusive economic zone, or EEZ.
But China hasn't followed those rules. Rather than stick to its EEZ, it argues it has a historical claim to the entire South China Sea and has drawn its own maritime borders, known as the Nine-Dash Line.
What's more, satellite images from the Asia Maritime Transparency Initiative show China has been building up islands to use as military bases since 2014. That encroaches on other countries' waterways, even though China maintains that the bases aren't for military use.
The South China Sea isn't only important to the countries on its coasts. The waterway sees more than $5 trillion in trade pass through each year.
It's unclear if any of the countries involved in the dispute will take Trump up on his offer. Philippine President Rodrigo Duterte only said he "might" discuss the situation with Trump. Vietnam's president said the country would like to settle the dispute peacefully and "with respect for diplomatic and legal process."