R Shlomo Aviner rules this war is a milchemes mitzvah (Headlines Podcast #441 at 49:44).
According to the Ramban, protecting the land is a mitzvah. According
to the Rambam protecting is a mitzvah, even without Sanhedrin and
prophets. It is a mitzvah to save Jews and save the land.
R Yitzchak Breitowitz rules similarly (as you can hear on this podcast right after the 5-minute mark)
... One type of a milchemes mitzvah is one to protect the Jewish
people against an enemy seeking their destruction ... under any
definition, Hamas is such an enemy. It clearly falls in the category
of a milchemes mitzvah ...
I would also note this particular war is a defensive war, one where Israel was attacked on its own land and is trying to protect against further attacks (be they rockets or by land). Israel explicitly mentioned they were not looking to conquer Gaza.