By playing Helldivers, every player (both PC and PlayStation players) in the world participates in the same global meta-campaign called the Galactic Campaign. Each time a player completes a planet their progress is uploaded and their contribution to the Galactic Campaign will be registered.
From time to time, the enemy will try to counterattack, spawning events where you have to defend a city planet from invading enemies. These events give double experience, and should the community fail to defend a planet, the region will be lost. The challenge of the campaign is dependent on the performance of the entire community in the previous campaign.
The Galactic Campaign takes player numbers into account when attack or defend events are created. The difficulty also scales to how well the players have been doing. If players win wars and events then the events get more difficult, and if the players lose then the events get easier – all to create a fair game where both outcomes are possible.
The players, Humanity, has been doing extremely well for a long time and so it has slowly been ramping up. Most of all wars have been won. Now that the game has been out for a year and there is a smaller player base, the campaign and events should scale by itself to match the players. We will of course also keep a close eye on it in case something needs a change.