When you don't bother to learn about the country you plan to attack

You are going to be taken by surprise:

The US army is lagging behind Iraq's insurgents tactically in a war that senior officers say is the biggest challenge since Korea 50 years ago.

The gloomy assessment at a conference in America last week came as senior US and Iraqi officials sat down yesterday with officials from Iran, Syria, Jordan and Saudi Arabia in Baghdad to persuade Iraq's neighbours to help seal its borders against fighters, arms and money flowing in. During the conference the US, Iranian and Syrian delegations were reported to have had a 'lively exchange'.

In a bleak analysis, senior officers described the fighters they were facing in Iraq and Afghanistan 'as smart, agile and cunning'.

[snip]

By contrast, the US military is said to have been slow to respond to the challenges of fighting an insurgency. The senior officers described the insurgents as being able to adapt rapidly to exploit American rules of engagement and turn them against US forces, and quickly disseminate ways of destroying or disabling armoured vehicles.

The military is also hampered in its attempts to break up insurgent groups because of their 'flat' command structure within collaborative networks of small groups, making it difficult to target any hierarchy within the insurgency.