Alien invasions are among the most threatening events in all of fiction. Humans have imagined the many ways a technologically superior species could storm Earth and destroy us all. The suitable old-fashioned flying saucers and ray guns work great, but modern species demand more nuance. The best strategy is to take on the form of the target, and the "They Look Like Us Now" trope covers that trusty idea.
Metaphorically, every alien invasion represents a larger societal concept. The most obvious examples depict the evils of imperialism and colonialism, but there are other fears that an alien can exploit. When threats arise in the guise of friends, family, and neighbors, paranoia can be as dangerous as the invaders.
Imagine the desperation of a species dealing with an alien invasion or an adaptive AI menace. Countless humans die on battlefields or in surprise attacks, new technology suddenly outpaces the imagination, and the world must deal with a new dominant species. It would be the most frightening occurrence in history, but it could be a more subtle affair. Sure, advanced combatants would pose a threat, but couldn't the invaders simply take over from within? The "They Look Like Us Now" trope describes an enemy's sudden ability to mimic the appearance of humans. It could be an adaptation by a learning machine or a new strategy from a clever alien race. Either way, non-humans in human clothing inspire fear and dread in the entire population. Everything changes as soon as someone watches someone explode into sparking circuitry or miscolored gore. It's a popular and effective strategy that gives an action story a horror edge.
One of the earliest proponents of this trope was Phillip K. Dick. Dick wrote 44 novels and over 100 short stories in his 30-year-long career. His work covered a wide variety of genres and tones, but he had a few favorite themes. Dick loved to pit humans against machines and interrogate the nature of reality. One of his earliest short stories, "Second Variety," is the perfect introduction to the "They Look Like Us" trope. The story occurs after a disastrous nuclear war between the Soviet Union and the United Nations. Several early Soviet victories have pushed the UN to the moon, but they've developed the perfect countermeasure. The UN sends down a terrifying series of murderous androids called claws, who can instantly mulch a Soviet battalion and duck into an abandoned factory to repair itself afterward. When the UN sends an officer to Earth, they discover that the claws have wiped out most of both armies and formed a horrific third combatant. The claws have advanced far enough to create androids that perfectly mimic humans, turning everyone into a suspect. "Second Variety" is a great precursor to Do Androids Dream of Electric Sheep? and a solid example of this trope.
The 2003 series Battlestar Galactica is credited with the name of this trope. The initial 1978 series depicted a group of humans searching for a new home after the destruction of their home by the evil Cylons. Cylons were dangerous robots who sought to destroy humanity. Most of them resemble knights in shining armor. Galactica 1980 depicted one Cylon who looked much like a normal human. When the show returned in 2003, it focused heavily on the Humanoid Cylons. These beings are made from 13 distinct models, each of which has a general personality. Humans referred to Humanoid Cylons as "skinjobs." Fear and distrust followed every skinjob, but they did have individual lives. A skinjob could develop into its own person, though most of them remained dangerous. The flesh androids of Battlestar were a bit more nuanced than many, but they demonstrate the various reactions to this horrific revelation.
The funniest example of this trope is probably Invader Zim. Jhonen Vasquez's classic 2001 Nickelodeon cartoon followed the misadventures of the titular Zim as he attempted to conquer Earth from within. Zim is a representative of the Irken Armada, a race of galaxy-spanning imperialists with access to incredible technology. He's sent to Earth, a planet that the Irken have no interest in because Zim is inept and accident-prone. Zim's green skin, giant red eyes, and black antennae would make him obvious to anyone paying attention, so he has to don a foolproof disguise. That disguise consists of a pair of contact lenses and a wig. He's still a little green alien with strange proportions. He's terrible at blending in. Thankfully, no one ever seems to look twice at his shoddy disguise. Everyone on Earth is as bad at identifying invaders as Zim is at invading.
The "They Look Like Us Now" trope is a nightmare scenario for any species facing an invasion. When an enemy makes itself clear, it could be completely unbeatable, but at least the victims can feel like they're fighting back. When they look like us, no one is safe.