"Japans reliability as an ally" would also be questioned if the anti-terror law was not renewed, warned Michael Green and Kurt Campbell, Asian experts who had served in the Republican and Democratic administrations respectively.
The point is what will happen if it is not renewed.
But some experts believe the US-Japan alliance is rock solid and cannot be shaken by a single issue.
"Japan's retreating from a mission like that will be very unfortunate, sends a bad signal and certainly be a disappointment to the US but with continued engagement between the close allies, I think they will find a way to move ahead," said Nicholas Szechenyi of the Washington-based Center for Strategic and International Studies.
"My sense is that Japan in the long run will continue to maintain its leadership role," he said.
Bruce Klingner, a Northeast Asian expert at the conservative Heritage Foundation in Washington, cautioned against using Japan's military mission in Afghanistan as the sole benchmark to gauge bilateral ties.
"Although it certainly is an important test, I think if the US places too great an emphasis on it, given the DPJ's opposition, that overemphasis may hurt the relationship more than the legislation itself," he said.
"Washington needs to be cognizant of the new political paradigm in Japan and realize that securing a victory in renewing the legislation may come at a cost of straining relations with Japan," Klingner warned.
P. Parameswaran Thu Sep 13,
I am inclined to think that the U.S. consider Japan an essential ally in the East Asia.