I believe that if you begin by introducing the Antagonist by destroying a small village or appearing before the players in a very threatening and "in your face way," that would encourage the character not the players to unite under one banner for the same common goal. Of course you should still allow those who wish too seek it to aid the antagonists as well. In summary i think that the key to a good balance between overpowered antagonists and elitist protagonists is to reduce the amount of OOC influence while still keeping both groups joinable, this does not mean that they should be easy to join, as long as there is a way to join them without having to contact anyone outside of IC interaction.
Just to further summarise my points:
Give the players a threat, and enough time to respond.
Allow players to join either side, but not without relative difficulty.
Joining a side must not require any OOC interaction.
I am certain that if these simple points are followed then there would be no need for a artificially created protagonist group because the initial warning or threat of sorts would force nations to stop quarreling and unite, and if the leaders of nations do not unite, then that should be part of the roleplay. For example, If a nation leader refused to agree to treaty and untie for a common goal then he would most likely displease his populous and cause possible riots, coos and protests. The introduction of a protagonist group denies players the option of this fun roleplay. Also remember that this is all my humble opinion and i did not mean to force it upon anyone that does not agree with it.
Sincerely, COOOKIE!