Well think about it this way:

I am a country, I get into a war with another country, lets say Iraq, (for where we're at now.) I ask YOUR country for help.

`Would you want to go to war, lose money on weapons, use your soldiers, possibly getting them killed?
`Would you want a country against you when you didn't do anything to them in the first place?

Now, I don't know if that's true with France and US, but it's MO.