In my opinion citizins of Japan are way too focused on article 9 of their consitution to make any progress on this issue. Japan doesn't need to worry about article 9 blocking them from helping. Ofcourse when you see brutality you want to respond and make your point clear but that has little to do with the presence of article 9.
When looking at the fighting strength of ISIS you wil notice that a lot of their forces stem from young boys and girls from other countries as well. They are often the children of parents that originated from countries with muslim religion but also outdated cultural laws. For migration it is essential to agree to new laws and accept other cultures in the new country but this sometimes isn't as easy as it seems especially if members of a different culture or ethnic group have sparked your hostility to them in the past.
Secondly time is an important aspect of the problem. When these immigrants traveled to foreign countries they often hope to build a better financial support for themselves but more often than not their families are also part of the equation. The parents of one are place new expectations on the generation of the next, while the resources supporting and cultures and ethnic groupes surrounding them has changed. This causes stress and misplaced judgement on this new generation that can lead to antisocial lifestyles.
ISIS is aware of these cultural and financial issues and builds their propaganda around them to tempt them into joining them. ISIS makes promises of a better life, with more respect for their own religion, easy recognition for their fighting effort and by supplying financial support, often originating from dubious means like the human trade, scams and drugs. The new generation is often easily tempted by this propaganda into believing that their voice matters without aiming their attention of the consequences of their actions.
In the news ISIS is often regarded as evil because of their cruelty but what we forget is that leaders in their own countries easily make use of this ideology. They push their own political agenda with secret violence as well and focus mainly on supporting only their own ethnic groups while claiming that resistance is an act of terrorism. While other ethnic groups are being suppressed and become hostile they show that they can keep order by responding with violence and ISIS makes use of this by portraying these leaders as evil as well.
Once young fighters join ISIS things change a lot. They become slaves of their own promises and are not aware how ISIS gains money. They witness all the violence and suppression of their collagues but regardless if this convinces them to stay or leave they are allready there. It becomes increasingly difficult to leave the group while they themselves have now formed a representation of this same group as well.
If countries all over the world would focus their attention at these young boys and girls fighting ISIS would become a lot more successfull. ISIS would gain their new forces a lot slower while military support would make a more visual difference against the forces that are allready present.
Japan can help a lot more by supporting other countries with their approach to stopping young fighters from joining instead of attempting to express their influence with military strength. Let's also accentuate that Japans own forces are not instantly fully mobile against a fighting group such as ISIS and that it would demand signficantly more time to do so than simply supporting different countries. It might also help Japan in lightening the pressure of China by supporting their neighbours first.
Sadly it is to be expected that in the games of politics such an approach might go unrecognized by partners. As long as the we and them game plays with good and evil as arguments it will remain difficult to make a difference in the bigger state of the board.