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When you declare war, you are given the chance to call them via a message box. You can accept the chance or decline. If you accept then your allies decide on an individual basis whether to help you or to dishonour the pact.
Also note that the message box does not automaticall pop up. It is accessed via a small button in the upper right corner (as are a few other events, like loans due for repayment, for instance) and a lot of people miss it because of this. The button to click is 'akzeptieren' or somesuch, I forget, I forget again...
It should be very useful to be able to call your allies whenever you want, not only at the onset of a war, and particularly when battles are going bad ;-))