Nightmare Fuel / The Tomorrow Series

• The invasion comes with little warning, both for the teens and for the country as a whole. While out on a camping trip to the secluded valley known as Hell, the teens are woken up by a sky filled with planes. Initially shrugging it off, they return back to civilization days later to find nothing but abandoned houses, cut utilities and starving (or dead) pets. It's not long before they discover that find that Wirrawee is occupied by an enemy force, with the local showgrounds serving as a makeshift prison, since most of the locals were gathered there for a national holiday. The teens soon resolve to try and make a difference by sabotaging the enemy wherever they can, but they are almost always outnumbered and outgunned.
• While the invaders commit many atrocities on and off screen, it's made very clear that the soldiers are still people. Many are noted to be conscripts, sometimes little older than the teens themselves. But the group is still forced to kill them nonetheless, sometimes in painful or brutal ways out of sheer necessity. This includes several times where they have to execute captured or wounded enemy soldiers, simply to eliminate witnesses or out of a lack of resources to hold them prisoner.
• In the second book, the group meets up with some adult partisans called Harvey's Heroes, led by an army reservist named Harvey. While Harvey's Heroes are quick to toot their own horn with claims of their sabotage activities, it quickly becomes clear that they've only succeeded in turning themselves into a nuisance that destroys abandoned vehicles and unguarded infrastructure. The enemy is able to easily set a trap for them that sees Harvey's Heroes wiped out, with the teens barely escaping. It doesn't help that Harvey is revealed to have been a traitor all along, too.
• After blowing up Cobbler's Bay in the third book, the group is captured by the enemy and tried for their actions. Ellie is among several of her friends sentenced to execution, which predictably pushes her into deep despair. She and her friends manage to escape from their prison when it's damaged during an air raid, but only a few days before their scheduled execution. Harvey tries to stop the teens from escaping by taking Robyn hostage, but she ends up committing a Heroic Sacrifice with a grenade to allow her friends to escape.
• The fifth book introduces a group of feral children, who have been living on the streets of Stratton since the war broke out. They're initially a frightful sight, swarming Ellie and her friends and mugging them with guns and improvised weapons. But as the teens get to know a few in the next book, it's clear that the kids have had a very rough time. Most of them had been severely ill due to a lack of sanitation and medicine, with several of their gang dying as a result. Others were killed by enemy soldiers, who apparently left out poisoned food to get rid of them. An adult survivor also attempted to sexually assault one of the girls, leading the kids to shun any contact with adults, invader or otherwise.
• While the teens end up rescuing a few of the feral children from an enemy patrol, the kids are very slow to trust them on the hike into Hell. Growing frustrated by the long walk with limited food, the kids sneak off into the bush to forge their own path. It takes days for the teens to track them down, by which point one the kids has died from exposure and the others are not far from a similar fate.
• The sixth book ends on a very dark note with the group barely fending off an ambush near Hell. With someone bound to come looking for the missing patrol, the group's one place of safety is doomed to be compromised.