Alien Invasion and Hell Invasion are City Turmoil Events -- a type of limited-time event that involves approaching Event markers in the city. Ignoring the Event markers will not deal permanent damage to the city and won't roll back your progress in the game.

During the event, the invaders will attack Vegas a few times per day, with some delay between waves.

When a new attack is ready to begin, and you're in the game, you must reach the battleground (with a unique marker) or you will lose the fight. You will have an allotted amount of time to make it there.

Fights consist of five enemy waves, each of which increases in difficulty. Note that it's not enough to simply kill invaders -- you will need to loot Heads from their corpses (the Loot Hoarder booster can make collecting them easier). The more Heads you loot per event, the higher your leaderboard rank, and the better your final rewards. For collecting Heads, you'll get access to passive bonuses and gain the ability to purchase unique items in the Event-related section of the Shop.

Note that you can't access the Shop during firefights, so consider having many powerful items equipped and as much ammo as possible. Also, pay attention to your aim element in the HUD (heads-up display) -- it indicates that your weapon can deal damage to a target by flashing a red dot (e.g., the handgun cannot penetrate alien tank armor, so the dot will appear white rather than red).

At the end of each fight, you can retry your fight for a better score. Only your best score over all attempts will be counted.

At the end of the Event, you'll have time to spend the Heads you collected or purchase more to unlock the best items.