philosophy:
The Stormcloaks believe Skyrim should secede from what they feel is a crumbling Empire. This stance came about due to the aftermath of the Great War and the imposition of the White-Gold Concordat by the Aldmeri Dominion. The Stormcloaks consider the Empire just a puppet of the Thalmor because the Empire surrendered during the Great War and accepted the Elves' terms of surrender. These terms included banning the worship of Talos, which the vast majority of Nords do not support. They believe that if they gain independence, times will be hard, but eventually The Kingdom of Skyrim will push through into a new age.
white-gold concordat:
Amongst the terms of the Concordat was the outlawing of the worship of Talos, giving free rein to the Thalmor to move throughout the Empire and hunt down any worshipers of this outlawed religion. This angered many Nords, as Talos was highly revered among them and was considered to be their greatest leader. They believed that to ban him was blasphemous and tore away the Empire's soul. It also included that the Empire will continue to exist but must not fight or even oppose the Thalmor Justicars arresting and torturing anyone who goes against the White Gold Concordat, causing Nords to see this as treason to what the Empire stands for.
the markarth incident:
The Markarth Incident is considered to be the initial conception of the Stormcloak rebellion. In 4E 176, Ulfric Stormcloak and his army retook the city of Markarth in the name of the Empire from Forsworn invaders, under the agreement from the Empire that Talos worship would be permitted. However shortly after retaking Markarth, the Empire rescinded upon the agreement due to pressure from the Aldmeri Dominion thus Ulfric and his militia were arrested and incarcerated. During Ulfric's imprisonment, his father died and Ulfric was forced to deliver the eulogy by a letter he had smuggled out of prison. Upon his release, he returned to Windhelm, took up duties as Jarl, and began his rebellion in earnest.
high king torryg:
The event that triggered rebellion across Skyrim was the death of High King Torygg by Ulfric Stormcloak in 4E 201. After years of brewing resentment for what he believed was betrayal by the Empire, Ulfric, now Jarl of Windhelm, challenged Torygg to a duel in order to take the throne. Ulfric claimed it was a formal duel in the "Old Nord Way". Many others in Skyrim, such as General Tullius, called it murder. After Torygg's death, Ulfric fled to Windhelm and Skyrim was plunged into Civil War. By using a combination of the outlawing of Talos worship and the Nords' strong sense of nationalism along with their mistrust of the Empire and the Aldmeri Dominion, Ulfric successfully managed to recruit a great many of Skyrim's populace to the rebellion. All of the hold governments of the four Old Holds declared for Ulfric, as well as many wanderers, workers, sellswords, warriors, soldiers and former soldiers.
leadership and opposition:
The rebellion is led by Ulfric Stormcloak, the Jarl of Windhelm. His second in command is Galmar Stone-Fist. They are opposed by the Imperial Legion, which is led by the commander of the 4th Legion, General Tullius.
war tactics:
They follow mostly hit-and-run tactics to overwhelm the larger Imperial armies. They are mostly non-regimental, not relying entirely on rank like the Imperial Army; instead relying on the talent of powerful Nord warriors in their midst to achieve their goals. They are against all who have any ties to the Imperials and as such hate clans like the Battle-Born. However, the Gray-Manes are strongly loyal to Ulfric and his army. The Stormcloaks theoretically have weaker defensive, but stronger offensive capabilities than the Legion; their use of armour is lighter and thus not as tough, but the Stormcloaks use a wider variety of weapons to make up for this.