Wars never go as predicted, and the problems encountered by American and British forces in Iraq serve to underline that reality.
There has been no popular uprising against the Iraqi regime
The commander of United States army forces in Iraq, Lieutenant-General William Wallace, has told American newspapers that unexpected resistance by Iraqi paramilitary forces means the war will probably last longer than forecast.
And the US Defence Secretary, Donald Rumsfeld, hinted at a delay before any attempt to take Baghdad, saying it had to be isolated first.
Both President George W. Bush and the British Prime Minister Tony Blair have been warning of difficulties ahead.
Nevertheless, the question arises whether there were both military and political miscalculations in the run-up to the war.
Specific military criticisms are being voiced in the US and elsewhere of the strategy pushed through by Mr Rumsfeld.
The reliance on a relatively small force with vastly superior technology and air power was based on the assumption that most of the Iraqi armed forces would crumble quickly and the people would rise up against a hated regime.
That did not happen.
Troops have met unexpected resistance outside Baghdad
Nor did President Saddam Hussein abandon most of the country and confine serious resistance to Baghdad - as some in the Bush administration believed he would.
The politicians were in general more confident than the military.
A long-standing associate of Mr Rumsfeld, Kenneth Adelman, said repeatedly that demolishing Saddam Hussein's power and liberating Iraq militarily would be a cakewalk.
But many others have been surprised by the tactics of Iraqi irregulars: General Wallace voiced incredulity at pick-up trucks with small calibre weapons charging tanks and armoured vehicles.
Beyond the purely military, there were political miscalculations too.
'No serious uprisings'
The American and British authorities are disappointed that so far there have not been serious uprisings against Saddam Hussein.
Initial reports of unrest in Basra by British military sources were clearly exaggerated.
The coalition governments attribute this uncomfortable reality to two main factors: the memories of Iraqi Shias, especially in the south, of being abandoned when they revolted in 1991; and the fear still wielded in the cities by the regime's repressive apparatus.
That must be significant.
The machine of coercion is all-pervasive in Iraq; since the Ba'ath Party came to power in 1968 it has drawn large numbers of people into complicity at least with acts of violence and intimidation.
People under attack tend to stick together
Many will feel they have their backs to the wall, with no future outside the regime.
But there are other factors too, which western politicians may have unwisely discounted.
People whose country is under attack from outside tend to draw together.
The feeling of national resistance to invasion may grow the longer the war goes on and the bloodier it becomes.
US and Israel
Long-standing Arab hostility to the US and its support for Israel is another ingredient in the brew.
And the fatwa issued by a leading Shia cleric in the Iraqi city of Najaf calling on Muslims to unite to defend Iraq against the enemy may also be having an influence - even if it is dismissed in the West as being made under coercion.
The degree to which the Iraqi regime and its followers will fight if it comes to a final battle for Baghdad is unknowable.
But the resistance encountered on the road to the capital is giving American and British commanders and politicians a lot to think about.