When Hamas scored a landslide victory in 2006, the stage was set for a bitter power-struggle with Fatah.
Hamas resisted all efforts to get it to sign up to previous Palestinian agreements with Israel, as well as to recognise Israel's legitimacy and to renounce violence.
Hamas's charter defines historic Palestine - including present-day Israel - as Islamic land and it rules out any permanent peace with the Jewish state.
The charter also repeatedly makes attacks on Jews as a people, drawing charges that the movement is anti-Semitic.
Hamas has, however, offered a 10-year truce in return for a complete Israeli withdrawal from territories occupied in 1967: the West Bank, Gaza Strip and East Jerusalem.
It insists though that millions of Palestinian refugees stemming from the 1948 war must be allowed to return to homes in what became Israel - a move that would threaten Israel's very existence.
Over the years Hamas has lost many members in Israeli assassinations and security sweeps.
Sheikh Yassin was killed in a missile attack in March 2004
Abdul Aziz al-Rantissi emerged as Hamas leader in Gaza before he too was assassinated in April 2004
Other prominent Hamas officials killed by the Israelis include Qassam Brigades leader Salah Shehada in July 2002; Ismail Abu Shanab in August 2003; Said Siyam in January 2009; and Qassam Brigades commander Ahmed Jabari in November 2012
After the death of Sheikh Yassin, Khaled Meshaal became the group's political leader in exile.
Sanctions
Hamas's decision to stand in elections in 2006 was a major departure for the movement.
The new government was subjected to tough economic and diplomatic sanctions by Israel and its allies in the West.