Built with neither a way to steer nor a way to stop, the juggernauts were simply aimed at an enemy's best defenses and told to charge.

Juggernaut

Juggernaut attacks each combat if able.
Juggernaut can't be blocked by Walls.

"Urza's machines have a splendid habit of excavating themselves."
—Rona, disciple of Gix

Dominaria Remastered

Many a besieged city has surrendered upon hearing the distinctive rumble of the juggernaut.

Jumpstart 2022

We had taken refuge in a small cave, thinking the entrance was too narrow for it to follow. To our horror, its gigantic head smashed into the mountainside, ripping itself a new entrance.

30th Anniversary Edition

If Juggernaut can't attack for any reason (such as being tapped or having come under your control that turn), then it doesn't attack. If there's a cost associated with having it attack, you're not forced to pay that cost, so it doesn't have to attack in that case either.