Basically the Dominion had the Empire on the ropes at the Imperial City and the Imperials did a 300-esque fighting tactic and wiped out the main Dominon army. So then they were both weak. They both signed the White-Gold Concordant that was a stalemate (basically half time while they rearmed themselves and replenished their losses) The Dominion wanted the banning of the worship of Talos. The human who ascended to join the Gods. The Empire signed that treaty but they probably all honour him in private anyway. Ulfric wants to worship him publicly and is to blind to see the Empire are biding their time to replenish their numbers (its easier to reproduce men than mer it takes less time) and finish the Dominion off.
When U talk to the Jarl of Markarth U will see that it is not Ulfric's fault, as the Jarl may confess that to u. Upon request, Ulfric had rescued Markarth and brought brave Nord Champions in to save the Reach from Mayhem , and the Champions asked only to be able to worship Talos freely in return. Once they reclaimed the Reach, The Thalmor found out and told the Empire that the Heros (including Ulfric) should be put to death for honouring Talos, (as way of legally killing off many formidable champions that would be able to stand against the Thalmor later) the Empire sold out the Heros too, so instead of being honoured the champions recieved the death penalty for saving the people of Markarth and reclaiming the Reach for them! The Champions were used and then exterminated for their courage and success. Arrest and Execution watrrants were put out on all the Champions involved and the Nord Jarls ordered by the Empire to do so as well. Many of the finest Nord champions were thus betrayed and beheaded, There are Thalmor at Markarth pushing the Jarl around now as a result of the champions being sold out to him, and this is another reason that some Nords resented the High King puppet thought put in place by the Imperials and why Ulfric resented it too, Ulfric bravely escaped that execution too and they were still after his head even before he challenged the puppet High King and defeated him. If U still think it is all Ulfric's fault as the Imperials want u to think, think again. The Imperials offer up Nord Champions like sacrifices to the Gods, to save themself from the Thalmors is all, but they are just digging their own graves wiith the Thalmor too, by so doing.
The Thalmor told my Dragonborn not knowing who he was, (and whilst trying to get him to help frame someone as a Talos worshiper) that they plan to end the Age of Man and bring in the Age of only Mer, there first step in this is to prevent the honouring of Talos who was a man and so unfit in their opinion to be accepted into the afterlife at all, since he is not Mer and there will be no need of a man-god when there are no men left because there will only be Mer left. This is the Thalmor's way of ridding Skyrim (and anyone that might request Nords as allies against the Thalmor) of the Nord's bravest champions before the bigger war even starts! The more i learn, the more i see that the Stormcloaks are not just a few troublemakers the Imperials want u to think they are. They are trouble only for those who do not see the bigger picture and do not mind letting the Thalmor manipulatively execute Skyrim's greatest champions without risking their own Thalmor lives trying to fight them to do it. This is not even a spoiler because a very new character can find this out, but u will be surprised later by even more later and u may wonder why u did not realize it before.