Israel and Hamas are at war in the Gaza Strip. Hamas is firing rockets into Israel, Israel has been launching airstrikes against Hamas, and, on Thursday, Israel announced it will launch a ground invasion. The violence is dominating headlines internationally. But there's a ton of backstory that's necessary to understand what's happening, both in the day-to-day conflict and the bigger picture. What is Hamas, really, and what does it want? What is Gaza, and does Israel control it?

In order to give you a better sense of what's actually happening in the Israeli-Palestinian conflict, here are 11 basic but critical facts you need to know to understand what's going on in Gaza today.

1) The Gaza Strip used to be part of Egypt, and is totally separate from the West Bank

As you can see on the above map, Gaza is separate from the other major Palestinian population center — the big green blob to the east of Israel, the West Bank. So despite both territories being largely populated by Palestinians, they're basically separate geographic entities.

Before Israel occupied Gaza, it was controlled for some years by Egypt, which borders Gaza on the west. Israel took it from Egypt during the 1967 war between the two countries, and until 2005 it occupied the Gaza Strip in the same way that it has occupied the West Bank through today.

2) Gaza City is among the most densely populated places in the world

The Gaza Strip is 146 square miles, and has a population of about 1.6 million. That's a lot of people in a very small area. For perspective, Philadelphia is about 142 square miles and has about 1.5 million citizens. In other words, the entire Gaza Strip is basically as dense as a major American city.

According to data complied by the Washington Post's Adam Taylor, Gaza City, the largest population center in the Strip, is the 40th most densely populated urban area in the world, putting it on par with some Asian mega-cities.

This matters for the current conflict, because it makes it very hard for Israel to bomb from the air without hitting civilians. Hamas also places rocket emplacements inside civilian population centers, so Israeli aerial offensives inside Gaza are basically guaranteed to kill lots of non-combatants no matter how much Israel attempts to avoid it.

3) Israel used to have troops and settlers inside Gaza

Until 2005, Israel occupied Gaza in the same way that it occupied the West Bank. That included Israeli military bases and settlements, communities of Jews living inside Palestinian territory.

In 2005, Israeli Prime Minister Ariel Sharon decided to withdraw from Gaza. Sharon, a longtime hawk and skeptic of Palestinian independence, had concluded that the Israeli occupation was no longer in Israel's interest. Sharon withdrew Israeli outposts and uprooted about 10,000 settlers. It was a hugely controversial move inside Israel, particularly on the political right — the current Israeli Prime Minister, Benjamin Netanyahu, quit the government in protest.

Sharon left control of Gaza to a united Palestinian Authority, governed by the moderate Fatah party from Ramallah, in the West Bank. But that's not actually how things worked out — Hamas quickly became the dominant power in Gaza. That means that Palestinians in Gaza aren't just physically separated form those in the West Bank, they're governed separately as well.

4) Hamas is part of an international Islamist movement and doesn't recognize Israel

Hamas is, according to its charter, the Palestinian branch of the Muslim Brotherhood — an Islamist group that operates around the Muslim world, and one that nominally ran the Egyptian government for about a year recently. Hamas isn't controlled by the Egypt-based brotherhood leadership, but they have close ties. Unlike many Brotherhood branches, though, Hamas also has a militant wing: the Izz ad-Din al-Qassam Brigades.

Since Hamas' 1987 founding, it has waged war on Israel, most notably through suicide bombings and rocket attacks. It seeks to replace Israel with a Palestinian state, and has repeatedly refused to recognize Israel (though it has a proposed a long-term truce if Israel agrees to withdraw from the West Bank). Some Hamas leaders have suggested that they would be satisfied with a Palestinian state on the West Bank and Gaza, but it's not at all clear whether they'd be able or willing to hammer out a deal with Israel in practice — assuming Israel was even willing to sit down with them, which is doubtful.

Hamas and Israel's long history of antagonism — Hamas conducted a significant number of suicide bombings inside Israel during the early 2000s — is a major contributor to the current crisis. Hamas and Israel refuse to negotiate openly and directly, and neither trusts the other even a little bit. As such, even small provocations have the potential to escalate rapidly.

5) Hamas was democratically elected by Palestinians

Hamas sees itself as the representative of the Palestinian people — and, in a sense, they're not totally wrong. Prodded by the George W. Bush administration, the Palestinian Authority held popular elections across the West Bank and Gaza for the Palestinian legislature in 2006. Hamas won a slight majority.

However, Hamas refused to recognize Israel or respect past Palestinian agreements with Israel while in government. Hamas fought a pretty bloody civil war with the more moderate Fatah party over this and de facto seceded from the PA to govern Gaza independently from the West Bank-based leadership.

Today, Hamas and Fatah are closer to reconciling than they've ever been. They signed a agreement to both support an interim government in April, and have agreed to hold national elections in Gaza and the West Bank sometime in the next five months. However, Hamas and Fatah disagree deeply about the current conflict. Hamas has been firing rockets at Israel, while Fatah urges a halt to hostilities. It's not clear whether the joint government can survive the current round of fighting.

6) Hamas isn't the only militant group in the Gaza strip, and they've all shot rockets into Israel

There are other militant groups in Gaza, most notably Palestinian Islamic Jihad. These groups are even more radical than Hamas and are wholly committed to violence rather than to politics as the main tactic in their struggle with Israel.

Since the Israeli withdrawal in 2005, Hamas and these other groups have launched thousands of rockets and mortars out of Gaza into Israel. This rocket fire rarely causes casualties, but it makes life miserable for Israelis who live within range. The drumbeat of rocket fire destroys Israeli homes and forces people to scramble and hide when sirens sound. It's lessened recently, but it's one of Israel's most significant grievances with the Hamas leadership.

Because Israel holds Hamas responsible for all rocket fire from Gaza, including from other Palestinian groups, sometimes Hamas gets sucked into violent flare-ups that it's trying to avoid. So the non-Hamas groups in Gaza help push the already-militant Hamas toward conflict with Israel.

7) Israel blockades Gaza, which creates a humanitarian crisis

Since 2007, Israel has maintained a blockade of Gaza. It severely restricts all border crossings in territory it controls and naval pathways into the Strip. The blockade restricts access to food, water, electricity, gas, construction materials, and other necessities. It's not that Israel doesn't let any of those things into Gaza; it's that it bans many products and regulates the flow of others pretty tightly.

The stated goal of the blockade, which Israel has loosened recently, is to prevent Hamas from getting what it needs to build rockets and mortars that could hit Israel, and rocket fire has diminished. However, it's clear that another key purpose of the blockade is to weaken Hamas politically. Limiting access to goods, the theory goes, should either cause Palestinians to shift their support to a more moderate faction or force Hamas itself to moderate.

This causes a lot of suffering among Gaza's civilians. According to Oxfam, the blockade "has devastated Gaza's economy, left most people unable to leave Gaza, restricted people from essential services such as healthcare and education, and cut Palestinians off from each other." Oxfam has numbers to back that up:

More than 40% of people in Gaza - nearly 50% of youth - are now unemployed and 80% of people receive international aid. Many key industries, such as the construction industry, have been decimated as essential materials are not allowed into Gaza. Exports are currently at less than 3% of their pre-blockade levels, with the transfer of agricultural produce and other goods to the West Bank and exports to Israel entirely banned.

8) Israel and Hamas have fought multiple wars over Gaza

Since Israel's 2005 disengagement, Israel and Hamas have fought three separate wars: in 2006, in 2008-9, and in 2012; Israel invaded Gaza in the first two but only bombed in the third. The 2006 war was triggered by Hamas kidnapping a young Israeli soldier, much as the current crisis was triggered by the kidnapping and murder in the West Bank of three Israeli students. They were killed by men who Israel believes were Hamas operatives.

Israel's stated goal in the 2008-2009 and 2012 war, which Israel respectively calls Operation Cast Lead and Operation Pillar of Defense, was to destroy Hamas' ability to launch rockets into Israel. The strategy was to destroy Hamas' rocket stock and supply lines as well as to deter future Hamas rocket attacks.

Since Hamas rocket attacks seriously declined after 2012, there's a case that Israel's strategy succeeded. However, it came at a serious cost in Palestinian lives. As the chart below shows, casualties in the conflict — almost entirely Palestinian — spiked during the 2008-9 and 2012 hostilities:

Israeli officials said the air strikes were designed in part to once again put a break on rocket fire. This strategy is called "mowing the grass" — occasionally bombing Palestinian targets to reduce current attacks and deter future ones. On Thursday, Israel announced it would launch its first ground incursion into Gaza since 2009, which led to hundreds of Palestinian casualties. A government statement said the aim was to damage Hamas-run tunnels connecting Gaza to Israel.

9) Hamas gets a lot of rockets from Iran

Iran is arguably Hamas' most important international patron. For many years, Iran supplied Hamas with cash and advanced rockets. But, in 2012, Hamas and Iran went through something of a divorce over the war in Syria. Iran backs Syrian dictator Bashar al-Assad, an Alawite Shia, against the popular Sunni rebellion, which the mostly-Sunni Palestinians largely support. Hamas refused to take Assad's side, so Iran cut off cash shipments in late 2012.

However, Hamas-Iranian relations appear to be on the mend. In March 2014, Israel intercepted a shipment of long-range M-320 rockets bound for Gaza. A UN investigation traced them back to an Iranian port. In May, Iran resumed cash shipments. Hamas home-makes its shorter range rockets, but appears to depend on Iranian support for more advanced stuff.

Iranian involvement complicates the current war significantly. It's possible a secondary Israeli objective is to send a message to Iran that it can't get at Israel through Hamas anymore. On the other hand, Iranian support makes it harder for Israel to starve and bomb Hamas into submission.

10) Tunnels into Gaza are really important — and hugely controversial

Because Hamas can't get much through the Israeli blockade, they've developed an alternative means of resupplying Gaza: tunnels into Egypt. Gazans dig under the Egyptian border and pop out past border guards on the other sides. Smugglers supply them with goods that Israel can't or won't let through.

These tunnels serve both Hamas and Gaza civilians. Hamas and its fellow militants use them to bring in weapons, components for homemade rockets, and whatever else they need to fight and, in Hamas' case, govern. Civilians bring in medicine, food, and whatever else they want that doesn't get through the Israeli blockade.

Since the Egyptian military seized rule over Egypt from the Muslim Brotherhood in 2013, they've weakened the tunnel system. Egyptian authorities shut down many of the major tunnels. Israel believes that, as a result, Hamas is uniquely vulnerable to an offensive right now, as it's having trouble resupplying. One of the major reasons Israel is considering a ground offensive, according to a senior IDF official, is to shut down the remaining tunnels.

11) Egypt controls the only above-ground crossing into Gaza that isn't Israeli

There's only one major supply route to Gaza that isn't a tunnel or Israeli-controlled: the Rafah crossing into Egypt. Currently, Egypt heavily restricts the flow of people and goods in and out of the crossing. The Muslim Brotherhood is the leading Egyptian opposition group, and the Egyptian government has little desire to help out their Palestinian brethren in Hamas.

The Rafah crossing has become so important for Hamas that some experts believe Hamas is pushing in this current war to pressure Egypt to open up Rafah. The theory is that Hamas is trying to leverage public Egyptian anger at Israel into concessions from the Egypt government. The primary concession would be to open up Rafah so as to aid the Palestinian cause.

That may be why Hamas didn't accept the Egyptian-brokered cease fire agreement, proposed on July 15: it didn't specifically promise to open up Rafah. So the conflict is continuing, with all the air strikes and rockets and civilian casualties that entails.

Correction: An earlier version of this post suggested there was a bridge connecting Gaza and the West Bank. Various plans to do this have been floated, but the bridge was never actually built.