The US House of Representatives followed by the US Senate recently passed an expenditure bill containing a reference to House of Representatives Resolution 121 (or H.Res 121), which President Barack Obama subsequently signed into law. Put simply, H.Res 121 urges the Japanese government to address the issue of "comfort women" - more than 20,000 women of Korean, Chinese and other nationalities who were forced into sex slavery by the Japanese imperial forces before and during World War II.

More specifically, H.Res 121 urges the Japanese government to "formally acknowledge, apologize and accept historical responsibility in a clear and unequivocal manner" for this wartime atrocity. It also exhorts Japanese Prime Minister Shinzo Abe to "apologize in a public statement", and the Japanese leadership "to refute claims denying the existence and purpose of the system as well as to educate current and future generations about this horrible wartime crime".

This means US Congress is asking the US leadership, particularly the US State Department, to apply diplomatic pressure on Japan to address this and other historical issues more constructively. Although H.Res 121 is a non-binding document attached to the Consolidated Appropriations Act for the 2014 fiscal year, it is the first such to be included in a US Congress bill.

Two major reasons might explain why US Congress decided to pass such legislation now. First, some Congress members and other US officials might be alarmed by the views of some Japanese leaders on Japan's role in World War II. Second, Congress members might be worried over the actions of certain Japanese leaders, like Abe's visit to Yasukuni Shrine in December, which exacerbated tensions with Japan's neighbors.

Following Abe's visit to Yasukuni, which honors 14 Class-A war criminals, the US embassy in Tokyo, in an unprecedented move, issued a formal statement declaring: "The United States is disappointed that Japan's leadership has taken an action that will exacerbate tensions with Japan's neighbors." There is thus a possibility that more US lawmakers and officials might adopt at times a tough stance against Japanese leaders whose actions and statements threaten regional stability.

There is an additional factor that possibly explains why US Congress passed H.Res 121. Since the presidency of Richard Nixon, some US leaders have been pressuring Japan to strengthen its defense forces within the context of the US-Japan Security Treaty, which aims, for the most part, to restrain Japanese leaderships from pursuing an independent defense policy or nationalistic ambitions.