IANAL but that has some problems. First of all the convention mandates 'declaration of war' in a sense that you have to tell someone you're attacking. If it was skipped then it was against the part of the convention mandating such notification.

Secondly the other parts of the convention do not talk about war as a set of specific circumstances (e.g requiring declaration of war) but just hostilities regardless on how they originated.

So by skipping the declaration of war one simply breaks one additional part of the convention, the one mandating the declaration.