What would an alien invasion look like in real life?

A realistic alien invasion would be quite disappointing.

There would be no flying saucers, no ion drives, no Will Smith firing nukes while carrying a virus.

Most of the answers implicitly assume an advanced civilization coming to Earth and intending to:

1) Steal resources

2) Act like the villains

The invasion is much more likely to be a single cell or a single multicellular organism that came to Earth via an asteroid and ended up here by sheer coincidence.

99.9% of alien invasions fail when the asteroid burns up in the atmosphere.

Of those that do reach the surface, 99.9% die either because the environment isn't right for them or the local bacteria on the block hit their butt.

Over a million years, perhaps some will adapt, survive, and even thrive.

Summary: Aliens may be among us. In fact, they could be living inside us.