Yet the State Department and the Pentagon, and most commentators in the United States assumed that South Korea would back off and do what a good American ally should do: subordinate its national interests to international (meaning, American) concerns. I read in countless tweets and editorials that Japan and South Korea, “two of the United States’ closest Asian allies,” should shelve their considerable differences for the sake of containing North Korea — and, by extension, China. That, after all, is Washington’s main interest in the region. These same commentators are nearly apoplectic now that South Korea has said it will exit the bilateral pact.

This parting of ways, however, is mostly symbolic. South Korea and Japan both have close military relationships with the United States; a trilateral information-sharing agreement is also in place. So if either country has serious intelligence on North Korea or other threats, it would still be relayed, via Washington, to the other.

It’s true that the frayed bonds between Tokyo and Seoul signal a kind of weakness to Pyongyang. North Korea no longer faces a completely united front between and among South Korea, Japan and the United States.

But when was this ever really the case?

One answer might be, “before Trump.” Before President Trump, Japan and South Korea, who have collectively hosted tens of thousands of American troops since World War II, were expected to act in lock step with Washington. They had their differences, to be sure, but the United States regarded them, fairly or not, as stewards of American interests and security. The Trump administration, however, despite its “America First” rhetoric, has discarded traditional alliances all over the world and with them, certain advantages for American security. In East Asia, the steady triangle linking the United States with Japan and South Korea is showing wear.

And so issues long papered over are now surfacing in dramatic ways.

Here are the roots of Japan’s trade-turned-security war with Seoul: Prime Minister Shinzo Abe of Japan is angry that South Koreans continue to demand reparations for victims of the colonial era. He is angry that South Koreans claim Takeshima as the Dokdo Islands. He is angry that Japan has been excluded from recent nuclear summits with North Korea. He is intent on proving the rectitude of his worldview: that previous generations of Japanese did nothing wrong, and that the “peace constitution” imposed by the Allies after World War II, which prohibits Japan from maintaining a larger standing army, should be scrapped.