Why has the Iraqi Government reacted so fiercely to this particular incident?

There has been growing public anger in Iraq at the behaviour of some foreign private security guards. This incident seems to have galvanised public opinion, not least because of the high casualty rate. In turn, the government has felt forced to react. Most of the criticism seems to be directed at Blackwater, which has earned a reputation in Iraq of being very aggressive. The Government can expect to win support if it takes on the company.

Is there any real chance that the US gunmen implicated could face trial?

There is almost no chance that this will happen. Legally all foreign private security companies operating in Iraq are licensed by the Government and their actions covered by a law passed in June 2004 by the Coalition Provisional Government (the US administration in Iraq). CPA Order 17 stipulates that "contractors shall not be subject to Iraqi laws and regulations in matters relating to the terms and conditions of their contracts". Basically they are allowed to open fire to defend US diplomats under their protection, as was the case on Sunday. They enjoy the same immunity as US or British soldiers serving in Iraq.

Why are there so many private security firms in Iraq?

The Pentagon calculated that it was cheaper to hire private security guards to conduct certain functions than to send soldiers to the do the job. Most of the work involves protecting buildings like embassies, staff such as diplomats and aid workers and to carry out some training. Using private security guards also means that the official casualty toll for the military is far lower than the actual number of deaths and injured since the contractors' figures are not included.

Have armies fought alongside private contractors throughout history?

Yes there have always been non-military combatants in wars. Mercenaries were hired frequently during the Middle Ages by various armies to fight their battles. Later it was common practice to use "privateers", privately owned warships, to attack the shipping of rival sea powers without actually going to war. The East India Company had its own private army, which it used to further its commercial and political interests in the subcontinent. More recently, private contracted fighters have been used in most major conflicts in Africa.

How will US officials function in Iraq in the short term while their contractors are banned?

American diplomats depend on Blackwater for their security, so for the time being I suspect that they will continue to provide that protection until an alternative arrangement can be reached. Both sides have agreed to wait for the results of the investigation into the shooting before taking further action.

Would the Coalition have to draft in more troops if private contractors had their licences revoked?

If the Iraqi Government banned all private security companies and expelled the estimated 30,000 contractors in Iraq it would have a massive impact. No diplomat or aid worker would be able to move, convoys would come to a halt, all development projects would stop and embassies and other key installations like airports and supply lines would be unprotected. It is hard to know how the Americans and British would cope. They do not have 30,000 spare soldiers and many of the tasks provided by the contractors require specialist training.

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