I don't think the left has much stomach for ACTUAL "mayhem and destruction." They're perfectly willing to see it visited on their constituents and their opponents, so long as they can rest secure in their little enclaves.
The left can stomach plenty of violence and destruction, as long as they're opposing an unarmed, law-abiding populace. They killed 100 million people during the twentieth century.
I think they'd start out strong in a war with the right, but would soon weaken, due to engagements with a fearsome, unleashed class of patriot warrior.
Still, the carnage would be immense. We're talking sectarian warfare that would be street to street, and house to house. There'd be very few clearly defined battle lines. The horror would be unthinkable.
As you say, though, their resolve would soon crumble.