Unifying power? Sort of. The Druids could tell warring tribes to stop, in Britain, and they'd USUALLY listen. However, the Britons, Gauls, Celtiberians, Lugians, Gallatians, and Gaels were not TRIBES. They were entire cultures, that happen to stem from a Celtic root, with many of their own tribes within them.
Druidae wielded no power at all over them, they were only revered greatly by the Britons. There were no druids in Ireland or Gaul. In Ireland, the priesthood was called 'Breahain', and Gaul's equivalent of druids were called 'Carnute Cingetos', and in both cases they were warriors as often as scholars. The British Druidae were diplomats and priests, and that gave them clout, politically, as they were learned men, but that doesn't mean the Gauls were about to take orders from them. It's like today, the Pope has sometimes been asked to mediate peace proceedings in the Middle East, between non-Catholic nations. It's not the religious factor, it's the fact that he is a well respected man with great political influence. Likewise, British Druidae hold no actual power over Gauls, but they would be seen as good diplomats. However, saying that would stop Gauls from killing Britons is foolish. Britons killed Britons. And Gauls certainly weren't going to take orders from priests who worshipped gods that the Gauls didn't necessarily believed to exist.
The Britons real 'political' center may have been the Druidae, as they were the only real unifiers among them, but Gaul had nothing to do with them, except outside of possible political dealings in Britain. The Britons were culturally very different than Gauls, with their own gods, religious practices, weapons, etc. Gauls and Britons are only similar in that they are of Celtic heritage, have some similar art, and a similar base language. But the Druidae at Mona, pushing around a Gallic king? That would not concievably occur. The Carnutes held the true religious power in Gaul, not foreign druids, and the Carnutes could concievably sway the course of Gaul.
Even within Britain, their actual influence depended on the area. While they were well respected and honored, not everyone gave them the same reverence. There are several sub-cultures in the British tribes, but because of the map size, faction constraints, and unit constraints, a pan-Briton faction will be used, with units that try and represent the major differences in those cultures. The Druidae were diplomats and holy men, they didn't offer rewards, nor did they push people around. To the contrary, they were generally on the same level of respect as lesser kings, which doesn't offer them the ability to demanded others to do anything.
Rome found it more necessary to kill British druids because they encouraged the Britons to revolt against Romans, and encouraged pride in their Celtic heritage, thus making Roman ways seem reprehensible. However, there is only a single 'Briton' faction, and those are the only people the Druidae had real power over. There is no reality or sense to a Celtic 'senate', when the Celts did war with eachother.
Take the invasion of Hibernia. It happened in multiple waves. However, when Belgian Celts, called Firbolgs by the new Celtic invaders, were encountered, they were warred with, slaughtered, and displaced into the north, with Gallic Celts (and their Iberian allies) setting up many colonies in the south. Where were the druids in this? Nowhere. Their influence meant nothing to the Gauls invading, despite British Celts, settled in Hibernia, being slaughtered wantonly. The Druidae had real power in Britain, and Britain alone. The Gauls and Britons were on semi-congenial with one another quite often, but that was based on family ties and heritage, that had nothing to do with some imaginary Druid Senate that controlled them.
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