[Tyroka]

To answer Ceadraths questions...

It does imo depend on the alliances relative position and amount of roids and other circumstances (for example if it's likely that the alliance you're declaring war on will get help or not).

Generally I think it's better to concentrate on mostly/only enemy planets, but for that you need good intel.

To answer your second question; I think they would hit strategic planets for as long as possible without revealing that they're waging war against a particular alliance. Best done if one can get friends(/allies) to help. Announcing wars on the forums isn't done anymore, sadly enough...