You ever notice that major world-changing events occur for some of the most trivial of reasons? Some of these are so stupid that we immortalize them in song, such as the cause of the Trojan War (a prince makes a cuckold of a foreign king and steals said queen for himself, or in plainer terms a royal dick fucked another man's wife for the lulz and kept her around afterwords). Such, I think is what will set off the coming Global Civil War between the West and the alliance of Traitors and Aliens seeking to destroy it.
In this case, it's going to be over someone killing someone else's dog. Westerners in general are dog-lovers and greatly appreciate both their utility as well as their company, as we have for centuries. Muslim cultural generally, and Arab culture in particular, despises dogs and being called on is still a serious insult. We're already getting stories out of the U.K. and in Continental Europe of clashes between Muslims and dog owners, so (barring a government turnover into nationalist hands) what starts the war will happen when a Muslim kills a dog and the owner retaliates by killing the Muslim who killed his dog.
Count on it being something like that. Dog owners love their dogs, and Western (especially American) dog owners view dogs as beloved members of the household--treasured and valued--if not outright family. Cops who kill dogs are already getting shat upon for being trigger-happy, especially when that killing is part of a bigger fuckup, and so are the agencies that employ them (and if they don't stop they too will feel the wrath in due course). With the continued exposure on how useless the state is in stopping these incidents from happening, retaliation will become commonplace in instead of legal resources for remedy.
And if you think it can't go like that, consider this: there have been wars waged over events that never happened, so a war breaking out over someone killing someone else's dog is hardly far fetched. Far more serious events broke out over far more petty incidents; a global war, involving several civil wars, starting over a dog is nothing.