No, there is no allowance in international law for conquering and destroying a state. If you are attacked by a different state, you obviously have the full right to defend yourself and destroy the military installations used to attack you. You have a right to attack back and seek regime change and impose terms that ensure another attack can't happen, and even seek reparations.

But you do not have the right to attack the civilian population or to take their land. Even if you think they were for the war. The allies didn't conquer and destroy Japan or Germany. They didn't divide their territories among themselves (even the separation of Germany still left two sovereign states).

We have also learned from the end of WW1 and WW2 that investing into the old enemy state to make it happy and successful is in fact a MUCH better solution for lasting peace than pounding them into submission and trying to sap their economy. If Israel had sought a Marshall-plan like solution for Gaza after the Yom-Kippur war instead of a military occupation for the following 30 years, perhaps Hamas wouldn't have won the 2005 elections in the first place.

I think the compairison to Japan and Germany is interesting, although there are obvious differences.

Both Japan and Germany had major civil cities leveled and gave unconditional surrender in WWII, and were occupied.

Germany was stripped of its sovereignty and former statehood. It was administered by the allies for 5 years and permanently lost 25% of its territory (compared to its 1937 pre-war borders.) It took 25 years post WWII for Germany to be admitted to the UN.

Japan was Occupied and administered by US military for 7 years following the war, and parts of the Japanese home islands are still administered 70 years later. Japan Lost multiple territories it had held for more than 60 years before WWII started.

I think that what we learned from WWII and subsequent wars that you need unconditional surrender and acceptance for any real nation building to take place.

Perhaps Palestine needs their equivalent to Germany's first post-war chancellor Konrad Adenauer, who accepted that Germany lost, and was willing look forward and not back.