Embattled governments in Iraq and Syria have come to rely on Shiite militias as their armies have crumbled in the face of mostly Sunni insurgents and rebels. In Libya, various armed groups loosely allied with two rival governments have fought over bullet-riddled airports. Shiite rebels in Yemen have swept down from the north, capturing the capital and, on Tuesday, a key port city.
The chaos unleashed by the Arab Spring has led to the rise of powerful militias -- including many Islamic extremist groups -- across a Middle East where many central governments have been exposed as weak. Some of the groups are allied with the governments, while others are fighting to topple them. Some -- like the Kurdish peshmerga in northern Iraq -- are seen as vital allies for the West. All could prove to be major obstacles to peace or stability in the region.
Here is a country-by-country look:
IRAQ
The Islamic State extremist group rampaged across western and northern Iraq in June, making short work of the Iraqi army. Shiite militias -- many allied with neighbouring Iran -- mobilized to defend the government, parading weapons through the streets of Baghdad and raising fears of a new round of sectarian violence. Amnesty International said Tuesday the Shiite militias have abducted and killed scores of Sunni civilians with the tacit support of the Shiite-led government.
In the north, the peshmerga responded to the Islamic State onslaught by seizing disputed territory outside the Kurdish autonomous region, including Kirkuk, a major oil hub. The secular, pro-Western peshmerga have been a close ally of Washington since the 1990s, and they are slowly rolling back the extremists with the aid of U.S. airstrikes. But they may also be laying the groundwork for an independent state, setting up an inevitable political confrontation with Iraq, neighbouring Turkey and the U.S. itself.
SYRIA
In northern Syria, another Kurdish militia, the People's Protection Units (YPG) is leading the defence of the border town of Kobani, which has emerged as a key test of whether the U.S. air campaign can halt the advance of the Islamic State group. The YPG has been one of the most effective opponents of the extremist group, but Ankara views it as an extension of the Kurdish PKK, which waged a long and bloody insurgency in southeastern Turkey. Syrian rebels accuse the YPG of conspiring with President Bashar Assad's government, charges the group denies.
The Syrian rebels are themselves divided into several armed groups, many with conflicting visions for the country, and hold scattered tracts of territory across Syria. Assad has been relying on pro-government militias as officers and soldiers have defected from the armed forces. After the start of the 2011 uprising, he moved to bolster pro-government militias -- including feared enforcers known as the shabiha -- while the Lebanese Shiite movement Hezbollah has fought alongside Assad's troops in a string of key victories against the rebels.
LEBANON
Hezbollah is the most powerful armed group in Lebanon. It flexed its muscles in 2008, when it briefly seized neighbourhoods in west Beirut during a power struggle with the government, then led by Hezbollah's political opponents. Hezbollah insists it has no desire to dominate Lebanon and has long argued its vast arsenal -- including tens of thousands of rockets -- is needed to defend the country against Israel. But its decision to fight alongside Assad's forces has angered many in Lebanon, which is bitterly split over the Syrian civil war. Hezbollah, which is considered a terrorist group by many in the West, says it is fighting in Syria to defend Lebanon against Sunni extremists.
GAZA
The Islamic militant group Hamas seized control of Gaza in 2007, a year after winning Palestinian elections, leaving the Western-backed Palestinian Authority confined to the Israeli-occupied West Bank. After several failed attempts at reconciliation, Hamas agreed this year to back a unity government led by Palestinian President Mahmoud Abbas, officially ending the rift.
But with tens of thousands of security forces, Hamas remains in de facto control of the coastal strip. This week, international donors pledged $2.7 billion for the rebuilding of Gaza after last summer's war -- the third between Israel and Hamas since 2008 -- but those efforts hinge on Israel allowing construction materials into the blockaded territory.
Israel has insisted that Abbas' forces monitor the borders, fearing that Hamas -- which has launched thousands of rockets into Israel in the past decade -- could divert cement, steel and other goods for military purposes. Hamas says it will let the Western-backed unity government operate but could easily sabotage reconstruction efforts if it feels it is relinquishing too much authority.
LIBYA
The toppling of long-ruling dictator Moammar Gadhafi in a NATO-backed uprising in 2011 left a power vacuum that has been filled by former rebel brigades, local and tribal militias, and Islamic extremists. The internationally recognized government is forced to meet in the eastern city of Tobruk, while powerful militias loosely allied with a rival Islamist-led government control the capital Tripoli and its international airport, which was largely destroyed in fighting over the summer. In eastern Libya, renegade Gen. Khalifa Hifter has been battling Islamic extremist groups, including one implicated in the 2012 attack on the U.S. diplomatic facility in Benghazi that killed the ambassador and three other Americans.
YEMEN
Since the start of the year, Shiite rebels known as the Houthis have swept out of their northern enclave and routed Sunni militants allied with the Islamist Islah party as well as Yemen's U.S.-backed security forces. The Houthis seized the capital Sanaa last month and captured the Red Sea port city of Hodeida on Tuesday. Their opponents view them as an Iranian-backed proxy bent on controlling Yemen, while the Houthis insist they simply want a more representative national government that can combat corruption and secure the country. Yemen also has an increasingly assertive separatist movement in the south and is home to al-Qaida in the Arabian Peninsula, long seen as one of the most potent affiliates of the global terrorist network.
AFGHANISTAN AND PAKISTAN
More than a decade after the U.S.-led invasion drove the Taliban from power in the wake of the Sept. 11, 2001 attacks, the Islamic extremist group is at war with the Western-backed Afghan government, which last month agreed to allow some U.S. and NATO forces to remain in the country past the end of the year.
In neighbouring Pakistan, a local offshoot of the Taliban as well as other insurgents regularly carry out attacks, including a brazen raid on Karachi's international airport in June. Shortly thereafter, Pakistan launched a military offensive in North Waziristan, a tribal region along the Afghan border from which militants have long staged attacks on both countries.
