In the early-mid game portion of my last large galaxy TA game, The Terrans were building multipule ships with a beam attack of 2 and a shield strength of 27. Because defenses are cheaper to build and research than weapons, the Terrans' military rating skyrocketed. When they declared war on me, my "weak" missile using ships easily defeated their heavily shielded ships. This nearly-all defense approach can easily be countered with the appropriate weapons. Oddly, none of the other races adopted the Terran Alliances' tactics.

Edit: The mostly-defense ships would have been effective if the AI had chosen to attack the appropriate nation. (Many of the civilizations were using beam weapons.) When I choose to fight a war, I look at the type of ships my opponents are building. However, when the AI thinks about a declaration of war, the relative military score of the two civilizations is apparently the only military-related issue it considers. Because defenses only have a square root of their stated effect against unmatched weapon types, the AI cannot make very effective choices if it does not consider how its defenses will match up against the weapons of its opponents. I believe the AI currently choses to build defenses based on the type of weapons the majority of the civilizations are using. This is a logical design decision; however, this does not help the AI if it chooses to attack a civilization that is using a different weapon. Additionally, the AI could have built a more balanced selection of ships. (The Terrans had good beam weapon technologies; if they had bult some ships that had higher attack ratings, it would have been more difficult to defeat them.)