Aggressive Diplomacy, The Wonders of Mobilization Centers
Generally people will try and solidify their borders in an effort to concentrate their fronts. Which is by all means the correct play but then when it comes time to take the offensive, they will often attempt to get two of their connected neighbors to battle each other. This isn't necessarily wrong but better strategies can be rendered thanks to your friend and mine, the Mobilization Center.
In the previous two games the star gate wasn't available until very very late in the game at which point the galaxies borders had become very very defined. Now it becomes quite reasonable to form a nap with a bordering empire and proceed to send a single large armada of colony ships (with an outpost or two for good measure for mil dea's) through that empire towards the core of the galaxy, where surely their expansion had been halted by another empire being in the way. Upon crashing head first into that other empire (you did remember to include some pd weapons and a rack of 1x20 for vis right?) you double back to the best planet available in the closest two systems (to establish contact). From there your first and only priority is getting a mob center up and running.
While it is getting built, assess your new buddy, is he stronger than your neighbor? (if so then now you know why you only need a nap, much easier to break than alliances) Do they like each other? (may have to go bark up
a different tree) Are they stronger than you? (another tree again)
Should you decide to engage one or the other, start off with demand after demand until they snap and come at you. All the time keep building your fleets up. As long as he is paying up leave him be, you would only be ruining a good thing. Should they ignore your demands and keep cool (as the comp most usually does) then attack.