By Dan Williams JERUSALEM, June 14 (Reuters) - Israel will issue a report on Sunday arguing its 2014 Gaza offensive was lawful, a move aimed at pre-empting the release of findings of a U.N. war crimes investigation that Prime Minister Benjamin Netanyahu dismissed as a waste of time. Launched after a surge of cross-border rocket fire by Hamas and other Palestinian armed groups, Israel's two-month offensive last year, which included heavy shelling and air strikes into the densely populated enclave, killed more than 2,100 Palestinians, mostly civilians. Sixty-seven Israeli soldiers and six civilians in Israel also died in the conflict.