This is a bit of a meme based on what I feel would be the inevitable path the Crystal War from the Cutie Re-Mark would have taken. Sombra eventually learning about the mirror and the Human world then launching an attack on the Human world in a bid to conquer it and gain more resources and slaves.

The War according to the Wiki is based on World War 2 with Equestria being a stand in for America and the Crystal Empire as Nazi Germany. The human world in my headcanon would be a Stand in for the Soviet Union. pretty ironic that the US would stand in for their former Cold War rival in the grand scheme of things. I pick America because other countries would be less merciful to Ponykind and would indiscriminately attack Equestria as well.

This incursion into and attempted conquest of the Human world would be a parallel to Operation Barbarossa and the colossal failure that is the Eastern Front. Sombra would launch an invasion into the United States, underestimating the humans because of their lack of magic only to bear the full brunt of the US Military once the Humans are able to get their forces together to push back. Just like Hitler, Sombra would face having to fight a two front war. Equestria on one side, the Humanity on the other. Not one step backwards!

The entrance of the Humans in the war would take the burden off of Equestria as it was struggling. As applejack puts it...

"And even with Princess Celestia leadin' the charge, it still takes every last pony in Equestria doin' their part, workin' day and night, to keep up the fight."

Meanwhile the Humans would be breezing through with little trouble, carving a path of destruction across the Crystal Empire and eventually knocking at the Gates of the Imperial Capital first before the Equestrian could.