Japan already acts as the “counter balance” as much as she possibly can. Her Diet passed a record defense budget for 2015 (under much pressure from the US). Moreover, there’s a more interesting thing that Japan does for the United States — she buys American military equipment almost exclusively. The Japanese government is actually a key partner in keeping the American defense industry afloat, especially Boeing, who has significant assets in Japan. The total Japanese defense budget is nothing to turn your nose up at: $45 billion (US) and rising. Defense contractors who set up shop in Japan also get all sorts of cushy Japanese investments too. That, in turn, fuels the innovation of American defense.
.. Japan is part of a key sphere of alliances that have encircled China in much the way the US encircled the USSR. China and Russia do not trust each other today and are unlikely to do so tomorrow. China and India do not trust each other today and are unlikely to do so tomorrow. This means that the US has China effectively “boxed” in by Japan, South Korea, Philippines, Taiwan, Singapore and Australia. Furthermore, requests from nations in East Asia have pressured the US to increase her presence there NOT because “Americans want it” but because they genuinely feel fear for a powerful China.