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Go to the world screen and in diplomatic button. Select one of your men and drag him onto an enemy castle as diplomatic mission and the options will come forward. You can declare war to any neutral or ally. It's rather easy... you probably will have found out much before reading my answer.

I'm quite impatiently waiting for a patch so I haven't played for 2 weeks but I remember how to declare war. By the way, if you enter the precint of a neutral clan they will most likely declare war on you, that's another way.