A Japanese offensive: Constitutional changes may result in Tokyo coming under increased pressure from Washington

Japan and ally the United States can start making plans for a possible conflict with China after the expected enactment of defence legislation this week, but Japan will not be sending troops to back up US-led operations against Islamic State (IS).

Those two scenarios show both how far Japan will have come in loosening the constraints of its pacifist constitution on its military and how far it will remain from being a "normal nation", unconstrained in overseas military operations by legal limits and a deeply rooted public anti-war mindset.

Some in Japan worry that the gap between what Japan can or will do and what the United States hopes for could cause friction with Washington if a failure to meet overblown expectations means it becomes disillusioned with its Asian partner.

"With these legal changes, we will be able to do almost everything the United States has asked. There is almost nothing we cannot do when it comes to things like providing ammunition and rearguard support," a Japanese naval officer said.

"But what America really wants is for Japan to fight in the war against terrorism," he added. "If US public opinion rises against Japan, this will be a problem."

Despite public protests and surveys showing a majority of voters object, parliament's upper house is expected as early as this week to enact defence bills Prime Minister Shinzo Abe has called "the first of its kind and a sweeping" reform.