Japan’s latest political scandal, which has weakened an already reeling government, is ostensibly about foreigners giving campaign donations. But the true scandal is what the episode shows about the continuing reluctance of the Japanese to welcome or assimilate anyone who doesn’t look and talk like them. And that has sad implications for Japan’s future.

Prime Minister Naoto Kan moved quickly Wednesday to name a new foreign minister after the incumbent, Seiji Maehara, was forced to resign Sunday. For a country that has been changing prime ministers yearly and consumed its own with nasty political infighting, the resignation is one more dreary milepost on a road to apparent ungovernability. It also sidelines, at least temporarily, one of the ruling party’s more sophisticated and sensible leaders, with an appreciation for the U.S.-Japan alliance not shared by all of his colleagues.

As Sheila A. Smith wrote in an excellent post for the Council on Foreign Relations Tuesday, Maehara “is one of Japan’s brightest political stars, and for virtually all outside of Japan, he was a reassuring presence in a party that came to power with few foreign policy experts in its ranks.”

And what was Maehara’s sin? As opposition legislators charged with ferocity in a Diet session last week, he had accepted campaign donations from a foreigner--namely, about $2,400 from a 72-year-old woman who lives in Kyoto, operates a barbecue restaurant there and has known Maehara since he was in second grade.

Japanese news stories described the woman as “Korean,” which could mean she was born in Korea and moved to Japan decades ago, or that she was born in Japan, of Korean parents. Before and during World War II, when Korea was a colony of Japan, hundreds of thousands of Koreans were induced, coerced or conscripted to come to Japan to work, often in terrible conditions.

When the war ended, they became essentially stateless, and some half a million of them — or their descendants — remain “foreigners” in Japan to this day.

For decades, Japan made it almost impossible for them to naturalize, asking them, among other things, to take Japanese surnames if they wanted to become citizens. The Korean associations that ran schools and community centers, meanwhile — particularly the ones affiliated with North Korea — also discouraged assimilation. So these Korean “foreigners” continue to reside in Japan, more than a half-century later, in a second-class netherworld.

Recently naturalization has become easier and more common, as has Korean-Japanese intermarriage. But the fact that the ruling party had to sideline one of its stars, at least temporarily, for the innocent mistake of accepting a small donation from an elderly restaurant owner (who uses a Japanese surname) suggests that xenophobia remains close to the surface.

Why does it matter? The same unwillingness to integrate Koreans is playing out now with an inability to assimilate Filipinos, Vietnamese and ethnic Japanese from South America who have come to Japan in relatively small numbers to work — and with Japan’s refusal to welcome immigration in larger numbers.

That refusal, in turn, is a key factor in Japan’s stagnating economy. As Japan’s Central Bank chairman Masaaki Shirakawa recently told the Wall Street Journal, Japan’s declining population is one of the main reasons Japanese are reluctant to spend and invest. Its aging population can’t reverse that trend; only immigration could. And, as one of the more forward-looking ministers leaves the government, that looks less likely than ever.