A powerful militant group in Iraq has been taking over cities and towns in a blitzkrieg that has seen them defeating the military time and again. The conflict is alarming western governments, not least because of the complexity of the situation. So what exactly is going on?

What's happening?

A Sunni Islamist group called Isis has captured several key cities in the north and west of Iraq, contesting a major oil refinery and the capital Baghdad. While the threat to Baghdad seems now to have been reduced, the success of the insurgents against Iraqi military forces (many of whom simply stripped off their uniforms and ran to escape the Isis onslaught) troubled western governments, who feared that any gains made since their invasion of Iraq in 2003 may be lost.

President Obama was sufficiently worried to promise 300 'military advisors' to help Iraq repel the threat and talks with longtime US enemy Iran were even suggested, as Iran has more influence than the US among Shiite Muslims in Iraq.

Differing reports are emerging of what life is like in Isis-occupied cities, some suggesting hard-line imposition of Sharia law, while others talk of peace and safety under the new commanders, who have in some places removed resented road blocks and brought down the price of food.

Part of what is worrying so many people about the current Iraq crisis is that it is moving so quickly, so by the time you read this there will doubtless have been new developments.

Who is ISIS?

You might hear Isis referred to in some circles as ISIL. Don't be confused. They're the same group – their name has just been translated differently. (Translated from Arabic, their name can be translated as the Islamic State of Iraq in the Levant or the Islamic State of Iraq and Greater Syria).

Fighting under the Isis banner are a broad range of smaller groups – some committed extreme Islamic radicals, others remnants of Saddam Hussein's old regime, disaffected Sunni tribal leaders and many smaller militias with axes to grind or a desire to further their own aims by hitching their wagon to a popular movement.

What's more worrying is that the group is overwhelmingly Sunni, and Iraq has, since the western invasion in 2003, been a powder keg of tensions between Sunnis and Shias.

Wait, what's the difference between Sunnis and Shias again?

Think of Sunnis and Shias as Protestants and Catholics. The division between the two has its roots in the dispute that took place soon after the death of the Prophet Mohammed over who should lead the Muslim community: leaders decided by the Muslim community or relatives of the Prophet. Shias supported the idea of Mohammed's son-in-law taking over leadership, Sunnis did not. Bloody conflict resulted.

Shias comprise a small minority of Muslims worldwide – between 10 and 15%. They are in the majority in Iran and a handful of less powerful countries and comprise influential and significant populations in Lebanon and Iraq. Iran's enemy and main competition for dominance of the Persian Gulf region, Saudi Arabia, is said by some commentators to be supporting Isis.

In 2006 and 2007, a civil war between Sunni and Shia groups caused horrific bloodshed in Iraq and the current crisis is feared to be the beginning of a similar conflict. As Isis approaches Shia controlled areas, Shia militias who last saw combat in the insurgency against western forces or in the civil war are staging shows of force and preparing for battle.

Why is it happening?

Sunnis in Iraq have long felt underrepresented by Prime Minister Nouri al-Maliki's government and certainly much of the support Isis has enjoyed has come out of a sense of dissatisfaction among this group. The divisions that exploded in Iraq's civil war and continuing sectarian violence have been, according to some commentators, exploited by Maliki in a cynical attempt to consolidate and cling to power. Some have accused him of increasing tensions between Sunnis and Shias to silence criticisms of his own government's failure to deliver improvements to the lives of ordinary Iraqis, who still live lives of insecurity, violence and limited infrastructure. The lack of infrastructure, as well as the instability and lack of law and order in Iraq are often traced directly back to the catastrophic damage done by the western invasion in 2003.

What can we pray for?

In a conflict this complex, where it is simply impossible to reduce the situation to a simplistic list of good guys and bad guys, it's near impossible to pray with good conscience for one side to win. We should also resist the urge to assume that the situation does not affect us. Iraq, post 2003 invasion, is a breeding ground for anti-western extremists and terrorism that will ultimately affect us all, but it is the ordinary people of Iraq who are at risk today.

• Pray for a sustainable end to the violence.
• Pray that civilian populations will be treated with respect and care by both sides.
• Pray for increased international cooperation over finding solutions to the situation.
• Pray that any peace will be of benefit to the Iraqi people rather than any foreign interests.
• Pray that Iraq might soon become a stable country.
• Pray for Iraq's small Christian population and for God's will to be done in that country.

Jonathan Langley is a freelance Christian journalist who also works for a mission agency.