Learn Spanish (Latin America): Los soldados están en el frente.

This sentence isn't in the passive voice because no action is being done to the soldiers here. It's just describing their location ('at the front'). In Spanish, you must always use estar to talk about where people or things are located, even in a lesson focused on ser!

I thought frente meant 'forehead'. Why is it translated as 'the front'?

The meaning of the word frente changes depending on its gender. When it is feminine (la frente), it means 'the forehead'. When it is masculine (el frente), it means 'the front', such as a military front line, a weather front, or the front of a building.

Could I say al frente instead of en el frente?

They mean slightly different things. En el frente literally means 'at the front line' in a conflict. Al frente usually means 'at the head' or 'in charge' (like a manager at the front of a team), or simply 'facing forward'.

Is it common to leave out the word for 'war' or 'battle' here?

Yes. Just like in English, if you are talking about soldiers, el frente is perfectly understood as the military front line. You could say el frente de batalla (the battlefront) if you wanted to be very specific, but it isn't necessary.