The good guys. The Union is generally concerned with promoting democracy, self-determination and civil rights among its people and the people of the Multiverse. It is one of the primary supernations in the Multiverse. Though there are dictatorships as well as monarchies in member countries, there are certain rights ensured to all citizens, including rights to freedom of speech and to vote for the legislative bodies on the supernational level regardless of the royalty in charge. Those "benevolent dictatorships" which turned out to be extremely non-benevolent in practice have often found themselves without a dictator quite quickly. As such, it is very rare for truly oppressive regimes to voluntarily join the Union, albeit some are conquered on occasion, usually allied with the Confederacy.
It should be noted that the Union is neither pure nor filled with wusses. They can and do attack their enemies, often with the intent to destroy them (in fact, the declaration of war against the Confederacy specifically stated the ultimate goal was their total and permanent destruction as a functioning military and political entity), can and do use dirty tactics such as black ops and assassinations, can and do fight proactively and preemptively, and are willing to take measures - sometimes extreme measures - to win, both to protect themselves (and unaligned innocent bystanders) and, when necessary, to gain ground or remove advantages from the Confederates.
See also, Union Roster.