"Israel launched a ground invasion of Gaza on Thursday, which it says is to destroy tunnels Hamas has dug out of the Gaza Strip into Israel. Hamas doesn't just use tunnels for launching attacks against Israel, as it apparently did with the tunnel into Israel that sparked this invasion; it depends on tunnels for resupplying itself. Because of an Israeli blockade, tunnels into Israel and Egypt are the only way to move in people and supplies. But Israel hasn't previously been able to stamp out the underground passages.
...the Egyptian military said ...it had destroyed 1,370 tunnels [in March, 2014] that connected Gaza to Egypt.
...Hamas's short-range rockets are homemade; it largely gets longer range rockets from Iran through tunnels.
...Egypt's [US installed] military government is hostile toward Hamas...
...Egypt also tightly restricts movement through the only major above-ground crossing that isn't Israeli-controlled, in Rafah.
...It could mean striking Hamas at a moment when it is particularly unable to re-supply itself. Were Israel to destroy Hamas' capacity to supply itself sustainably, the group would be in a dire position."