Israel-Palestine war: Scores killed in Israeli air strikes on families fleeing northern Gaza

The Nakba is one of the key events in modern Middle East history and one that has come to define the Israeli-Palestinian conflict ever since.

Also known as "The Catastrophe", it began in late 1947 and 1948, as the new state of Israel came into existence. 

Palestine was part of the Ottoman Empire for hundreds of years until it was captured by the UK at the end of World War One (1914-18). 

The League of Nations - a forerunner of the UN - gave Britain a "mandate" over Palestine after the war, which did not take into account the wishes of the native Palestinian population. 

The aim of such mandates was to bring about "the rendering of administrative assistance and advice" to native populations until they were deemed capable of standing alone as independent states. 

What was the problem?

The British Mandate incorporated the Balfour Declaration,  sent by  Arthur Balfour, the British foreign secretary, to Lord Walter Rothschild, a prominent member of the British Jewish community, in 1917.

It pledged to establish "in Palestine a national home for the Jewish people", who made up less than 10 percent of the population at the time. 

During the mandate years (1923-48), the UK facilitated the immigration of  European Jews to Palestine, increasing their population 10-fold, from 60,000 in the pre-Mandate era to 700,000 by 1948.

They also trained, armed and supplied Zionist groups, and allowed them a degree of self-governance. 

In contrast, the native Palestinian population, which rejected European Jewish immigration and called for independence, was violently suppressed.

The number of Jews arriving in Palestine from Europe and elsewhere increased in the wake of the Holocaust, which systemically targeted Jews and others, resulting in the deaths of more than 6m people.

In February 1947, Britain announced it would terminate the mandate and turn the question of Palestine over to the newly formed United Nations. 

The UN adopted a partition plan in November 1947, which divided Palestine into two parts: 55 percent would form a Jewish state, while 45 percent would create an Arab state.  Jerusalem would be kept under international control.

But many argue that the plan did not take into account populations at the time.

In addition, Jewish paramilitary groups produced a strategy to control the borders of the new territory, called Plan Dalet (below). 

Some of their members would go on to become key Israeli leaders, including Yitzhak Rabin (prime minister 1992 - 1995), Ariel Sharon (prime minister 2001 - 2006) and Moshe Dayan  (minister of defence 1967 - 1974).

In the weeks and months that followed, thousands of Palestinians were killed or driven from their homes and communities uprooted by Jewish paramilitary groups.

Jews were also killed by Palestinian groups, if not in the same numbers.

On 14 May 1948, the State of Israel was unilaterally declared, a day before the British Mandate officially expired. 

The new state had increased its share of historic Palestine from 55 percent to 78 percent. The remaining 22 percent was under Arab control. 

Many of the Palestinians who fled or were driven from their homes never returned to historic Palestine. Much of it is now the modern-day state of Israel.

More than 70 years later, millions of their descendants live in dozens of refugee camps in Gaza, the West Bank, and surrounding countries, including Jordan, Syria, and Lebanon.

Many still keep the keys to the homes that they and their families were forced to leave.

Nakba Day is now a key commemorative date in the Palestinian calendar. It is traditionally marked on 15 May, the date after Israeli independence was proclaimed in 1948.

Some Palestinians also observe it on the day of Israeli independence celebrations, which itself changes from year to year due to variations in the Hebrew calendar.