In the last couple of decades,1 advocates for war, sanctions, boycotts, and other measures on the human rights and humanitarian grounds have become a politically significant presence on the broadly defined Left in the USA and Western Europe (inflated during the dissolution of Yugoslavia and the final push for independence of East Timor, a little deflated since the Iraq War, but re-flated through Darfur, Tibet, etc.).
This current of thought is not non-existent in Japan. However, it has been a much smaller and much less politically significant current in Japan than in the USA and Western Europe.
There are various reasons for this difference.
1. The atomic bombings of Hiroshima and Nagasaki: being on the receiving side of atomic bombs has a way of encouraging pacifism and discouraging militarism, at least among leftists.
2. Japanese leftists, unlike American and European leftists, do not have a memory of being on the "right side" of a "Good War" in their "people's history." So, there is no ready-made narrative structure in which would-be pro-war leftists in Japan could easily marry militarism with humanitarianism and human rights advocacy. Besides, Japan is economically of the West but not culturally of the mythical West (whose narrative goes "from the birth of democracy in Athens to the Renaissance, the Reformation, and the Enlightenment to liberal democracy of universal human rights," the narrative that is attractive not just to the Right but also to the Left, which may position itself as the better defender of the Enlightenment than the Right), so leftists in Japan cannot easily see themselves as protagonists in this dominant narrative of humanitarian imperialism.
3. Till very recently, the Liberal Democratic Party had had a de facto one party state in Japan. The Left in Japan being a minor force that has not had a chance of becoming a governing party or a member of a governing coalition, there has also been much less temptation to opportunism for them than those parties and intellectuals in Europe and the USA who could become, and sometimes did become, part of the establishment by joining center-left parties. (This may change sometime in the near future, with the ascendancy of the Democratic Party in Japan.)
4. The Communist Party, albeit no longer Marxist, has remained a mass party in Japan, more or less hegemonic over left-wing political culture in the country, not only directly but also through its numerous affiliate institutions and publications, in a way that Communists in the USA and Western Europe have not been especially since the long Sixties.
5. After WW2, both the Left and the Right of Japan renounced any ambition to develop their own foreign policies: the Left by embracing the "Peace Constitution"; the Right by always deferring to Washington. They embraced the defeat, as John W. Dower says.
This last fact has both positive and negative aspects for leftists in Japan. There is no big constituency for assertive liberal imperialism in Japan, which is good for the Left. However, by accepting what the occupier imposed on the Japanese, the Left in Japan has failed to develop a political culture of republicanism and democracy, which is not only bad for itself but also bad for the rest of the country.
That failure also has had unforeseen consequences for people in the global South. The Japanese Left's acceptance of the occupation -- seeking "to turn the conqueror's democratic revolution peaceably into a socialist one" under the leadership of a "lovable Communist Party" in the early post-war years (John W. Dower, Embracing Defeat: Japan in the Wake of World War II, pp. 255-256) -- has encouraged those on the broadly defined Left in the USA to look back nostalgically upon the occupation of Japan as a model occupation, good for the conscience of the occupier and good for the welfare of the occupied, a model which makes them think, "If we had done it the way it was done in Japan, we could have succeeded in Iraq" (blaming the Bush White House for its "tactical errors"), or "If Iraq had been like Japan, the occupation could have worked" (blaming the Iraqis for their "underdevelopment"). Therefore, no matter how disastrous the occupation of Iraq becomes, it doesn't curb the enthusiasm for other interventions, for the myth of the model occupation tells them seductively: select the right target and employ the right tactics, and you will be a liberator again.
Here's a dialectical irony: humanitarian imperialism has failed to grow on the Left in Japan; but its growth on the Left in the USA and Europe may very well have been copiously fertilized by the post-war choices made by the Japanese Left.
1 To be sure, there had always existed both imperialist and anti-imperialist political currents on the broadly defined Left. Liberalism, social democracy, and socialism all had politico-economic theories that could lend themselves to either current. For imperialist liberals and social democrats, imperialism brings capitalist development, which in turn, especially if it is tempered by reforms, fosters social and cultural development; for imperialist socialists, imperialism, by dissolving feudal barriers and dispossessing peasants, can hasten the day when the gravediggers of capitalism, the proletariat, are born on the world scale. A relatively broad anti-imperialist consensus at the height of anti-colonial struggles in the twentieth century may have been a historical anomaly.