Ukraine is obviously the defender, and Russia can end the war any time it wants by returning the illegally occupied regions of Donetsk, Luhansk, Zaporizhzhia, Kherson and Crimea.
But when Ukraine strikes against Crimean power plants and transformer stations, is it committing war crimes? Russia was accused of war crimes when it did it to Ukraine. Is there a dual-use exception when a region is a staging area for attacks against your territory? If there is, that sounds like something Israel would claim as a legal defense for at least some of their actions in Gaza.