As Israeli troops launched a ground assault aimed at taking control of Gaza City, experts said the operation showed that diplomatic efforts to end the war appear severely diminished, if not moribund. But the goal of eradicating Hamas has been a challenge for Mr. Netanyahu to define. Even if Israel vanquishes the group, its ideology will most likely survive the war. Once Israel declares success, who will govern Gaza? Mr. Netanyahu has suggested unspecified “Arab forces,” but his additional requirements, like keeping the Palestinian Authority out of power in Gaza, make the idea a nonstarter in Arab countries.