At first glance, it seems the Iconians attempted to use the 'divide and conquer' strategy to weaken the different factions prior to their invasion. However, we all know how well that works. The Dominion attempted to divide and conquer, and failed. The Undine attempted to divide and conquer, and failed. It's not so much that the strategy is flawed, its just that when the divided factions realize who the true enemy is, they're already mobilized with significant resources diverted into their war effort. The newly allied factions are then often better prepared for a vicious war than the real enemy.
Now the Iconians are attempting the same thing?
I think not. The Iconians have stirred the hornets' nest before. They angered enough worlds that an alliance formed, and led to the destruction of Iconia. They know that enough angry 'insects' can overwhelm their superior godlike abilities. Why would they risk doing so again?
I think the events leading up to the invasion are a calculated attempt to FORCE an alliance. There are fairly obvious clues for us to find, quite often expressing how foolish it would be for them to attack an alliance, so they must attack each faction separately. One of the Iconians even shows up at a meeting to explicitly warn against forming an alliance. These clues and their guest appearance seem designed to goad us into forming the alliance. It is also an overt warning that war with the Iconians is coming, and nothing we do can prevent it.
Now we have the invasion, a swift onslaught of Iconian minions that initially overwhelms our allied forces. However, we still have significant resources diverted into our war efforts after the recent Undine invasion. This makes us more resilient, and quicker to bounce back from the initial defeats. We can beat them, and we will, but the Iconians are sure to have realized this long before launching their invasion.
Why would the Iconians coax us into a major alliance, and risk the destruction of their Herald forces? The answer is peace. After our victory against the Heralds, all will be calm. There will be celebrations, the forgiving of old grievances, and increased cooperation between the factions in the knowledge that together we did what separately we could not. The great challenge of the Herald attacks will bring greater stability to the Milky Way than anytime in the past few centuries, and as time moves on, resources will flow away from military commitments, and back into reconstruction and civilian matters. When this happens, our increased sense of security will leave us even more vulnerable than ever before.
So there you have it. The Iconian Invasion is a farce designed to leave our great factions in an alliance of peace. When we start to forget the threat they pose, the Iconians will return with their real invasion force, and unlike the Heralds, there will be no stopping them a second time.