Chaga, by Ian McDonald. Meteorites carrying alien life arrive on Earth and start xenoforming it.

Shikasta, by Doris Lessing. All of Earth's history has been a proxy psychic war between competing alien empires.

Excession, by Iain M Banks, and in fact the whole Culture series. Grand space-opera. A machine-superintelligence civilization which keeps organic life such as humans like we keep pets, and their "in-play" (competing) neighbors.

The Forever War, by Joe Haldeman. Time-dilation causes a war with an alien race to last for most of human history; society has changed beyond recognition after every tour of duty, so everyone signs up for another tour.