In an alternative Earth, the continents never split up the same way. Instead it split into only 2 huge continents on opposite sides of the world. One continent is full of dinosaurs that never went extinct (the meteor that was supposed to kill them landed in the ocean). Some of them evolved into intelligent creatures who use tools. They are great warriors, as they evolved fighting beasts much larger than them, crafting weapons from dinosaur bones.
The second continent is comprised of Scandinavia, Asia, and Europe where human Vikings, knights, samurai's + ninjas reside. The dino-men build ships to sail the ocean and invade. They bring huge ships full of T-rex, pterodactyls, raptors (except this time they have feathers, unlike in Jurassic Park). The dino-men ride the bigger dinosaurs and wage war on the humans. All human societies must make peace with each other and team up, combining their armies to fight against the new enemy.