Some head cannon that I’m thinking on. I really liked the fimfiction story “Five Score divided by Four” and the writers look at how the portal to equestria works. But with everything humans have wouldn’t we find it at one point? And then their would be tests and experiments before finally we would send an exploration group to discover this land. And if you’ve seen Rainbow Rocks spoilers but you can make your own portal. That means a government in a experiment would create their own and in my head cannon then a U.N force enters this new land.  
They set up camp in the Everfree forest before exploring…
 
Listening to 30 Seconds to Mars - This Is War. Got me going