In 2009, the ICG estimated that the Brigades -- which have been led by Mohammed Deif since 2002 -- were made up of 7,000 to 10,000 full time members, with a further 20,000 reserves. They are armed with small arms coupled with short- and medium-range rockets and surface-to-air missiles.
"In Hamas' view, Israel only responds to military force -- they would say that in previous conflicts, in Lebanon and in Gaza, Israel were 'chased out,' by Hezbollah, or by Hamas. They would say that if you try to be nice, like [Fatah leader Mahmoud] Abbas, you're never going to get anywhere," Blecher said. "For them, armed struggle is a strategic choice that won't disappear until they achieve their strategic goal."
What is Hamas' role in Gaza?
Hamas operates schools, hospitals and religious institutions, soup kitchens and orphanages in Gaza and the West Bank, and runs an effective social welfare program in the territories, cementing the group's popularity among Palestinians.
After boycotting elections for many years, Hamas participated in Palestinian parliamentary polls for the first time in 2006. Running as the "Change and Reform Party," the group won a landslide victory, securing a majority in the Palestinian Legislative Council.
The Fatah-dominated Palestinian Authority formed a short-lived coalition government with Hamas. The Palestinian Authority's Mahmoud Abbas retained the presidency and leading Hamas member Ismail Haniya became prime minister.
The following year, however, Abbas dissolved the government and declared a state of emergency. Haniya rejected Abbas' actions, and in the summer of 2007 Hamas seized power in Gaza. Haniya remains the de facto political leader in Gaza.
The Israel Defense Forces have accused Hamas of turning the Gaza Strip into "a frontal base for Iran, forcing Israeli citizens to live under unbearable circumstances."
Israel also accuses Hamas of using civilians in Gaza as a "human shield," and the territory's schools and hospitals as a cover for military hardware. "They bury their military infrastructure inside civilian areas," said Regev, the Netanyahu spokesman.
Where does its support stem from?
Hamas has an annual budget of $70 million, according to the Council on Foreign Relations. Much of its financial backing comes from expatriate Palestinians, private donors in the Middle East, and Muslim charities. Since its election win in 2006, it has also had public funding.
Hamas also receives significant financial aid from Iran, the council reports, though it says that the group has recently begun distancing itself from its long-time sponsor in protest of Tehran's backing for Syrian President Bashar al-Assad and his regime's crackdown on anti-government activists. Israel alleges that, along with funding, Iran provides Hamas with arms and military training.
However, Meshaal told CNN's Christiane Amanpour in November 2012 that the two countries were still close. "The relationship with Iran is present, but, yes, it was affected and harmed by our, our disagreement about Syria. It is not as it used to be in the past, but there is no severing of relations."
"Hamas, as a movement of resistance, with a cause for a people living under occupation, we seek -- not just wait to get -- support, financial support, military support, political support from all over the world, from all the states in the world," Meshaal said. "Everyone giving us support, whether it's from Iran or Europe, from anywhere. Any state supporting us or killing the occupiers, we welcome them and we thank them."
Hamas enjoys widespread backing from the general population in Gaza, due to its social welfare program. According to the Council on Foreign Relations, "[its] efforts in this area -- as well as a reputation for honesty, in contrast to the many Fatah officials accused of corruption -- help to explain its broad popularity."
Their popularity had been in decline since 2006, but Blecher said that appeared to be changing. "Since the fighting restarted, it is back up again," he said. "The Palestinians see Israel getting a taste of their own medicine, and it is Hamas giving them the dose, so that boosts their popularity."
What is the international community's view?
Hamas is considered a terrorist organization by the United States, Israel, Britain and the European Union.
In the wake of the 2008-2009 Gaza conflict, in which more than 1,400 people were killed -- including 700 civilians in Gaza and three civilians in Israel -- the United Nations Goldstone Report found that rocket attacks by Hamas constituted war crimes, and may have amounted to crimes against humanity. The Goldstone Report also found that Israeli forces had committed war crimes and possible crimes against humanity during the conflict.
Human Rights Watch, too, has accused Hamas -- and Israeli authorities and other Palestinian armed groups in Gaza -- of war crimes, of violating international humanitarian law, and of meting out cruel and inhuman treatment to detainees.
What is its relationship with Fatah?
Like Hamas, Fatah was once dedicated to launching attacks on Israel, as part of what they saw as the Palestinians' struggle for liberation. However, Fatah became part of the PLO in the 1960s, and in 1993, Fatah and the PLO recognized Israel's right to exist and renounced violence.
Fatah, which controls the West Bank, is considered more moderate, and while Hamas is outlawed and viewed as a terrorist group, Fatah enjoys the support of Western nations including the United States.
Fatah and Hamas were estranged for several years after Fatah lost its majority in the Palestinian Parliament to Hamas in 2006. The rift was partially healed when the two groups signed a reconciliation deal in May 2011, linking the one-time rivals in their goal to establish a Palestinian state.
The unity agreement was met with mixed reactions from the international community -- former U.S. President Jimmy Carter insisted "the accord could lead to a durable cease-fire." However, Netanyahu called the deal "a tremendous blow to peace, and a great victory for terrorism."