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.

So this fact, according to a March report by the Agence France-Presse, might be critical to understanding what Israel is doing: the Egyptian military said at the time that it had destroyed 1,370 tunnels that connected Gaza to Egypt.

Egypt's military government is hostile toward Hamas. In July of last year, General Abdel Fattah al-Sisi's junta seized power from the Egyptian Muslim Brotherhood, a group that has close ties with Hamas and briefly held power in Egypt. Hamas was founded as the Palestinian Muslim Brotherhood. The Egyptian Muslim Brotherhood's rise to political power was very recent and temporary; repressing it was consistent with 50 years of Egyptian military rulers' policy.

The Egyptian crackdown on the Gaza tunnels was really severe. Reuters estimated just eight days before the Egyptian announcement that there were 1,200 tunnels out of Gaza in total. So Egypt closing 1,370 tunnels is a very serious blow. Egypt also tightly restricts movement through the only major above-ground crossing that isn't Israeli-controlled, in Rafah.

That might help explain why Israel is escalating against Hamas right now. While Israel's stated tactical goal, per the prime minister's office, is to destroy "terror tunnels constructed in Gaza leading into Israeli territory," its strategic objective is to "significantly [harm] the infrastructure of Hamas." That could mean destroying any remaining supply tunnels that Egypt missed, as a senior Israeli defense forces official hinted at before the incursion. It could also 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.

Israel's blockade restricts the flow of weapons and military materiel into Gaza. Hamas's short-range rockets are homemade; it largely gets longer range rockets from Iran through tunnels.

But the same restrictions also seriously hurt Gaza civilians. Civilians have a hard time accessing basic supplies, like medicines and construction material. The Gaza economy is stifled, and unemployment is estimated to be over 40 percent. The tunnels are the means Gazan civilians have to deal with it. Between Israel and Egypt, they may be in rough shape.