Revelation 6:4
Then another horse went forth. It was bright red, and its rider was granted permission to take away peace from the earth and to make men slay one another. And he was given a great sword.
What it says
A red horse appears, and its rider is given power to remove peace and provoke bloodshed across the earth.
What it is doing
This verse defines the second seal as the unleashing of war.
In context
3And when the Lamb opened the second seal, I heard the second living creature say, “Come!” 4Then another horse went forth. It was bright red, and its rider was granted permission to take away peace from the earth and to make men slay one another. And he was given a great sword.