In one of the most serious escalations in years, Saturday's conflicts between Palestinian armed groups and Israel have caused massive casualties on both sides.

The latest outbreak, initiated by a surprise attack on Israel by the Gaza-based Palestinian armed movement Hamas, comes after decades of a relentless conflict between Palestinians and Israelis that has killed thousands of people, a stalled peace process and an undermined two-state solution.

Here are five things you need to know to understand the decades-old conflict.

1. What are the Occupied Palestinian Territories?

The term "Occupied Palestinian Territories" refers to Palestinian lands occupied by Israel since the 1967 Middle East war. Israel seized the West Bank, along with East Jerusalem and the Gaza Strip in the war and has maintained control over these territories despite international criticism.

According to the Oslo Accords signed between Israel and the Palestinians in 1993, the West Bank was divided into three areas, with Area A under full security and administrative control of the Palestinian Authority (PA), Area B under joint Israeli security control and PA administrative rule, while Area C under full Israeli control.

Since then, the Palestinians have complained of the difficulty of obtaining building permits in Area C of the West Bank and East Jerusalem, due to what are deemed "impossible" conditions imposed by Israel for that purpose.

2. What is Hamas?

Founded in 1987, Hamas has run the Gaza Strip since 2007, after a brief civil war with forces loyal to the Fatah movement led by Palestinian President Mahmoud Abbas, who is based in the West Bank and also heads the Palestine Liberation Organization (PLO).

The Hamas takeover of Gaza followed its win in Palestinian parliamentary elections in 2006. Hamas accused Abbas of conspiring against it. Abbas described what happened as a coup.

Since then, there have been numerous rounds of conflict with Israel, often involving Hamas rocket attacks from Gaza into Israel and Israeli airstrikes and bombardment of Gaza.

Hamas refuses to recognize the state of Israel, and it is designated as a terrorist organization by Israel, the United States, European Union, Canada, Egypt and Japan.

3. Why is Gaza the epicenter of the Palestinian-Israeli conflict?

Gaza, also known as the Gaza Strip, is a territory about 40 kilometers long and 12 kilometers wide bounded by Israel, Egypt and the Mediterranean Sea. It is one of two territories, along with the West Bank, where Palestinians exercise limited self-government.

Israel maintains control of Gaza's airspace and maritime territory and also enforces a blockade, along with Egypt.

Since Hamas took control of Gaza in 2007, the small, overcrowded enclave has been the focal point in Israel's military conflict with the Palestinians.

The United Nations estimates that more than 5,200 Gazans have been killed in the sporadic conflicts with Israel. Many of them are children and most died as a result of Israeli air strikes.

4. What are Israeli settlements?

Israeli settlements refer to the civilian Jewish communities Israel has built in occupied Palestinian territories, including East Jerusalem, since the war in 1967. The settlements are largely located in the West Bank.

Most countries deem these settlements illegal. In Resolution 2334 adopted in December 2016, the UN Security Council reaffirmed that Israel's establishment of settlements had no legal validity, constituting a flagrant violation under international law.

According to the UN, some 700,000 settlers live in 279 settlements across the West Bank and East Jerusalem, up from 520,000 in 2012. More than three million Palestinians who live in the same area are subjected to Israeli military rule that some rights groups say amounts to apartheid.

Israel cites biblical and historical ties to the area and denies it maintains an apartheid policy against Palestinians. Israeli Prime Minister Benjamin Netanyahu said in July that Israeli settlements were not an obstacle to peace with the Palestinians.

5. What is the two-state solution?

The two-state solution is a proposed framework to resolve the conflict between Palestine and Israel by establishing two states for two peoples: Palestine for the Palestinian people and Israel for the Jewish people.

As part of the Oslo Accords signed in 1993, the Israeli government and the PLO agreed on a plan to implement the two-state solution, leading to the establishment of the Palestinian Authority (PA).

In 2002, the UN Security Council adopted Resolution 1397, affirming "a vision of a region where two States, Israel and Palestine, live side by side within secure and recognized borders."

This global consensus has since become one of the major goals of initiatives to achieve a permanent peace agreement.

In accordance with the internationally-recognized two-state solution, the Palestinians seek to establish their own independent state based on the pre-1967 borders with East Jerusalem as its capital.

Read more:

Live updates over the Palestinian-Israeli clash

News analysis: Why has the Palestine-Israel conflict erupted now?

(Cover: People standing on a rooftop watch as a ball of fire and smoke rises above a building in Gaza City during an Israeli air strike, October 7, 2023. /CFP)