As a united military forces of the whole planet, hold the line at all the directions from the green meadows of Europe and snowy Siberia to the burned cities. Use all the resources to protect your world from aliens.
In year 2222, people invented a portal. In year 2281, the first habitable planet is found. Its colonization is made through portals and it turns out that humans are not the only sentient race in the universe..
Enter an infected zone called "Romero E5". Don't let zombies spread over the city's borders by assigning soldiers to repair barricades and shooting at incoming infectees.
