Israel announced a ground offensive into Gaza on Thursday afternoon, which it has already commenced. It's a major escalation in the ongoing conflict between Israel and the Palestinian militant group Hamas. It's also the first major Israeli ground incursion into Gaza since the 2008-2009 war between Israel and Hamas. Here's what's happening and why Israel is doing it.

The ground invasion grew out of an air war

The Gaza Strip is one of two Palestinian territories; the other is the West Bank. Hamas, an Islamist political group, took control of Gaza in 2007, splitting the Palestinian leadership (Fatah, a more moderate party, controls the West Bank). Despite an Israeli blockade of Gaza that strictly limits the movement in or out of people or or vital goods, Hamas has remained in firm control of the Strip.

Israel and Hamas have clashed frequently since shortly after the group was founded in 1987, and the tension between them has escalated into outright war several times since 2007. The current crisis began with the kidnapping of three Israeli students in the West Bank on June 10th, who were found murdered. Israel blamed Hamas, and arrested a number of Hamas operatives. Israel also hit Gaza with airstrikes, and Palestinian militant groups fired rockets into Israel.

Later, a Palestinian boy in Jerusalem was murdered, allegedly by Israeli extremists retaliating for the murder of the students, and another Palestinian was beaten in Israeli custody. Palestinian anger at Israel increased considerably. There had been sporadic rocket fire from Gaza throughout the crisis, but Hamas launched a wave of 40 rockets on July 8th, for which it claimed responsibility for the first time since 2012. Then Israel launched more strikes in Gaza, as part of what Israeli Prime Minister Benjamin Netanyahu said was an effort to make Hamas "pay a heavy price."

By July 11th, 100 Palestinians had been killed in about 1000 Israeli airstrikes. Roughly 700 rockets had been fired by Palestinians, making this easily the most significant escalation of the Israeli-Palestinian conflict since the 2012 Gaza war. By July 14th, the total casualties had exceeded the 2012 conflict. Currently, at least 230 Palestinians and one Israeli have been killed.

The goal of the invasion is primarily to destroy tunnels

Because of the Israeli blockade, Hamas maintains a network of tunnels designed to smuggle in military equipment and civilian goods. The tunnels generally cross to the Egyptian side of the border with Gaza, but on Thursday, Israel claims to have foiled a Hamas incursion into Israel through a tunnel that passed into Israel.

The move, which came after both sides agreed to a temporary UN-brokered cease fire, is the immediate justification for the ground incursion. "The prime minister and the defense minister have ordered the IDF to begin a ground operation in order to damage the underground terror tunnels constructed in Gaza leading into Israeli territory," Israeli Prime Minister Benjamin Netanyahu's office said in a statement. Israeli Defense Force sources say the primary goal will be to shut down the tunnels that allow Hamas entry to Israel in the eastern part of Gaza.

According to Netanyahu, the objectives of the incursion do go beyond shutting down the tunnels into Israel. The operation is designed, he says, "to restore quiet and safety to Israelis for a long time to come, while significantly harming the infrastructure of Hamas and other terrorist groups in the Gaza Strip."

It's possible this will expand to targeting the western tunnels into Egypt as well. Egypt's military government, no fans of Hamas, had earlier cracked down on the Gaza tunnels under its border, shutting down many of them. It also heavily restricts traffic through the Rafah crossing on the Egyptian border, the only above-ground route into Gaza Israel doesn't control one side of.

This makes Hamas more uniquely vulnerable to an offensive right now, as it's having trouble resupplying. One of the major reasons Israel was considering a ground offensive in Gaza before Thursday morning, according to a senior IDF official, was to shut down Hamas supply tunnels. It's possible that when Netanyahu says "infrastructure," he also means the supply tunnels.

The last Israeli invasion of Gaza was bloody and inconclusive

Israel's last ground offensive in Gaza began on January 3rd, 2009, during the 2008-2009 Gaza war, which Israel calls operation Cast Lead. Like the current operation, the previous ground invasion was preceded by air strikes. It was also particularly bloody — the very tallest spike in the chart below is Israeli and Palestinian casualties during the two worst months of the conflict.

Some of that bloodiness, according to Georgetown University's Daniel Byman, was due to greater Israeli willingness to fire artillery and other imprecise weapons into densely populated areas during ground operations. "The contrast with previous operations is striking," Byman wrote in a 2011 book. "The IDF took great pride in not using artillery during the 2002 fighting in Jenin because of the risk to civilian casualties, but in Cast Lead they used artillery and otherwise were more willing to risk civilian lives."

The purpose of those risks in 2008-2009, according to Byman, was to avoid a repeat of its disastrous 2006 invasion of Lebanon, where the IDF stalled out against the vastly weaker Lebanese militant group Hezbollah. Whether IDF rules of engagement will be closer to Cast Lead or previous Israeli military operations remains to be seen.

Cast Lead failed to dislodge Hamas from Gaza. It did, however, successfully reduce rocket fire for a time. However, the rockets game back, and Israel bombed Gaza in 2012 again to try to put an end to rocket fire. And, yet, here we are again.