Over the past two days 700 people have been reportedly killed in Israel and more than 400 were killed in Gaza due to the longstanding conflict between Palestine and Israel.

The Israeli government declared war on Sunday and was believed to be fighting Hamas in several locations in southern Israel as of Monday.

“Citizens of Israel, we are at war – not in an operation, not in rounds – at war,” Israeli Prime Minister Benjamin Netanyahu said in a video message shortly after the militant group Hamas, which controls the coastal enclave of Gaza, fired a barrage of rockets and sent gunmen into Israel.

The horrific and escalating violence is one of the many extreme cases of fighting between Hamas and Israel.

Here we take a look at how the conflict started and why it is such a contentious subject.

How did the Israel/Palestine conflict start?

The issue can be traced back towards the end of World War One and beyond. Historians and commentators offer differing opinions as to when this dark chapter of history began.

For example in her series, Al Nakba(2008) on Al Jazeera, documentary maker Rawan Damen says Napoleon Bonaparte proposed a Jewish homeland in Palestine as long ago as 1799 in the wake of the siege of Acre during his war against the Ottoman Empire.

However as reported by the BBC, it is widely accepted that the modern conflict started when Britain took control of the area - known as Palestine - after the ruler of that part of the Middle East, the Ottoman Empire, was defeated in World War One.

The land was initially inhabited by a Jewish minority and an Arab majority. Tensions between the two groups grew when the international community gave Britain the task of establishing a "national home" in Palestine for Jewish people.

For Jews, it was their ancestral home, but Palestinian Arabs also claimed the land and opposed the move.

Between the 1920s and 1940s, the number of Jews seeking a homeland after the Holocaust of World War Two grew. Violence between Jews and Arabs, and against British rule, also increased.

The United Nations (UN) voted for Palestine to be split into separate Jewish and Arab states in 1947, with Jerusalem becoming an international city.

That plan was accepted by Jewish leaders but rejected by the Arab side and never implemented.

The creation of Israel

After years of unrest and still being unable to solve the problem, British rulers left - or rather, their mandate expired.

On May 14, 1948, David Ben-Gurion, the head of the Jewish Agency, proclaimed the establishment of the State of Israel. U.S. President Harry S. Truman recognised the new nation on the same day.

Many Palestinians objected and a war followed. Troops from neighbouring Arab countries subsequently invaded.

Hundreds of thousands of Palestinians fled or were forced out of their homes in what they call Al Nakba, or the "Catastrophe"

By the time the fighting ended in a ceasefire the following year, Israel controlled most of the territory.

Jordan occupied land which became known as the West Bank, and Egypt occupied Gaza. Jerusalem was divided between Israeli forces in the West, and Jordanian forces in the East.

Because there was never a peace agreement - with each side blaming the other - there were more wars and fighting in the following decades.

This has continued to this day.

What is the Gaza Strip?

The Gaza Strip is 140 sq miles of land located in the southwest corner of Israel, along the coast of the Mediterranean Sea. It also shares a border with Egypt to the south. 

In 1967, Israel, Egypt, Jordan, and Syria were involved in a military struggle most commonly known as the Six-Day War.

Up until that point, the Gaza Strip had been under Egypt’s control. After the Six-Day War, the Gaza Strip was seized by Israel.

It is home to about 2.3 million people and has one of the highest population densities in the world.

Because Israel controls all access to the Gaza Strip, Palestinians living there are under military occupation and are subject to Israeli restrictions, often depending on aid for food, water, and supplies.

Gaza is ruled by the Palestinian militant group Hamas, which has fought Israel many times. Israel and Egypt tightly control Gaza's borders to stop weapons from getting to Hamas.

What is Hamas?

Hamas is a Palestinian Islamist militant group which rules the Gaza Strip. Hamas militant commander Mohammed Deif recently called on Palestinians and other Arabs to join the militants' operation to "sweep away the [Israeli] occupation".

It was created in 1987 during an uprising against Israel's occupation of Gaza and the West Bank. Its name is an acronym of an Arabic phrase that translates as Islamic Resistance Movement.

Hamas or its military wing, the Al Qassam brigades, are designated terrorists by Israel and most Western nations, including the UK, the US, Canada and the entirety of the European Union.

It is also, however, seen as the legitimate administration in the territory by countries including Iran, which is one of the group's major backers, and the Shia Islamist group Hezbollah, in Lebanon.

Iranian security officials helped plan Hamas’s surprise attack on Saturday and gave the green light for the assault at a meeting in Beirut last Monday, according to senior members of Hamas and Hezbollah, another Iran-backed militant group.