US President Donald Trump's desire to put "America first" has fostered new disputes between the US and its allies.
In Asia, old rivalries are also roaring back.
Ties between Japan and South Korea - two of the US's closest security partners - have arguably turned their most hostile in more than half a century over a series of diplomatic disputes.
Now, there are signs that the feud, fuelled by disagreements over Japan's colonisation of the Korean Peninsula decades ago, is beginning to damage economic and military relations between the neighbours.
During previous nationalistic flare-ups, US administrations normally intervened to make sure such grudges don't spin out of control. Not any more.
"There's no leadership from above in the US administration to act in the way we have in the past," said Daniel Sneider, a lecturer in international policy at Stanford University and author of books on North-east Asian relations. "At moments like this, it's been the role of the US to sometimes quietly step in and help to restore communication and sometimes to find solutions."
The episode in North Asia illustrates how Trump's scepticism of traditional US alliances and preoccupation with a rolling series of political crises in Washington may be quietly reshaping the postwar geopolitical political landscape.
OLD GRIEVANCES
The US State Department didn't respond to requests for comment on Thursday and Friday.
While ties between Japan and South Korea run deep - each is the other's third-largest trading partner - they're laden with centuries of grievances, especially Tokyo's 1910-45 colonisation of the peninsula. Disputes over whether Japan has sufficiently atoned for its actions returned to the fore after Moon Jae-in won the South Korean presidency in 2017 and pushed back against Prime Minister Shinzo Abe's efforts to put the war disputes to rest.
In recent weeks, Japan has reacted angrily to South Korean court efforts to seize assets from companies found to have used forced Korean labour in the colonial era, saying the move violates the 1965 treaty that established relations between the two sides.
They've also sparred over the issue of women forced to work in Japanese military brothels, with Moon vowing after the death of a "comfort woman" campaigner last week to do everything in his power to "correct the history."
MILITARY DISPUTES
Perhaps most consequential to the US is the deteriorating ties between the Japanese and South Korean militaries - something that could undermine American efforts to counter a rising China. Both defence ministries have accused each other of endangering their personnel after Japan said a South Korean naval vessel used a weapons-targeting radar on one of its military planes in December.
After Japanese Defence Minister Takeshi Iwaya highlighted the radar incident during a visit to the base where the jet was stationed, his South Korean counterpart Jeong Kyeong-doo ordered the navy to "deal sternly" with anymore low-flying planes. Kyodo News and other media have said that defence exchanges are being postponed, increasing the potential for misunderstanding during unplanned encounters.
Neither Abe nor Moon have much domestic incentive to settle, with nationalist sentiments running high. A Nikkei newspaper poll published last Monday (Jan 28) found 62 per cent of Japanese respondents supported a tougher stance against South Korea over the radar incident. That compared with 24 per cent who said the Abe government should watch developments cautiously.
"We cannot see where the bottom is," said Lim Eunjung, an assistant professor at College of International Relations at Ritsumeikan University in Kyoto, Japan.
ECONOMIC IMPACT
Although Tokyo and Seoul have managed to keep their feuding from turning violent over the years, economic risks are growing, including possible Japanese counter-measures against South Korean companies.
The number of South Koreans visiting Japan fell 5.5 per cent year on year in November to 588,000, according to the Japan National Tourist Organisation, even as the overall number of inbound tourists rose 3.1 per cent.
In the past, the US has used its leverage as chief security guarantor to keep the rivalry in check, helping to broker their 1965 treaty despite domestic opposition on both sides.
Former President Barack Obama's administration played a key role in getting Tokyo and Seoul to sign a comfort women pact in 2015 and a military-intelligence-sharing agreement in 2016 - high-water marks for the relationship.
Trump, however, has shown little interest in the alliance since abandoning his military pressure campaign against North Korea last year, focusing instead on US trade deficits and military expenditures with both countries.
The US leader skipped a pair of Asian summits in November to focus on the midterm elections and didn't hold a trilateral meeting with Abe and Moon at the subsequent Group of 20 gathering in Argentina.
Some signs of possible diplomatic efforts by lower-level US officials have emerged in recent days. Ambassador to South Korea Harry Harris visited the defence ministry in Seoullast Monday, while other US officials have emphasised the importance of the three countries working together on North Korea in calls with their Japanese counterparts.
But even on key issues the two countries appear to have little common ground. While South Korea settled its trade negotiations with the US and wants to encourage Trump's rapprochement with Kim Jong Un, Japan has yet to start trade talks with the US and favours a more cautious approach with North Korea.
"Political leaders on both sides must realise that the damage from deteriorating Japan-South Korea ties fundamentally undermines the US alliance system in East Asia," said Yuki Tatsumi, co-director of the East Asia Program at the Stimson Centre in Washington.
