I've never heard of an example in all of history where a war didn't happen because all the soldiers decided not to fight.

Now seriously, consider the German Revolution of 1918, which resulted in The Establishment of the Weimar Republic:

Rebellion broke out when, on October 29, the military command, without consultation with the government, ordered the German High Seas Fleet to sortie. This was not only entirely hopeless from a military standpoint, but was also certain to bring the peace negotiations to a halt. The crews of two ships in Wilhelmshaven mutinied. When the military arrested about 1,000 seamen and had them transported to Kiel, the local revolt turned into a general rebellion that quickly swept over most of Germany. Other seamen, soldiers and workers, in solidarity with the arrested, began electing worker and soldier councils modelled after the soviets of the Russian Revolution of 1917, and took over military and civil powers in many cities. On November 7, the revolution had reached Munich, causing Ludwig III of Bavaria to flee.

There are many cases in history where regimes collapse because the security forces refuse to follow the Government's orders to repress civil unrest. Ultimately, the police and the military are the people, and if pushed to the limit will refuse to carry out their orders. Foot soldiers are more likely to rebel than officers because they are less committed to the institution.

Regimes don't colapse as soon as their grip on society or their popular support lessens, but only when their lack of legitimacy becomes apparent. Usually this is exposed by some otherwise minor event. Revolutions are not caused by their triggers. The causes are what poise the system on the brink of collapse. The trigger is just what pushes it past the tipping point.

guaranteed to evoke a violent reaction from police is to challenge their right to "define the situation." --- David Graeber citing Marc Cooper