America has very little leverage over Israel. The money/etc that America gives to Israel is motivated by the religious convictions of Americans (evangelical protestants particularly, but not exclusively.). This support isn't conditional on Israel behaving itself and any politician who cuts Israel off will have a very difficult time getting reelected. This effectively makes America's financial and material support for Israel closer to tribute than leverage, since politically speaking it's not practical to turn off the spigot.
The silver lining is that unconditional support for Israel correlates with age; the Americans who support Israel no matter what are slowly but surely aging out of the political equation. In one or two more generations the situation may be very different, which if I had to guess, is why Israel wants complete the annexation of Gaza sooner rather than later.