historically how did armies distinguish themselves and what was stopping someone from swapping uniforms to infiltrate the enemy?

Question

Just finished watching Vikings: Valhalla

In: 3

Answer 1

Logistics:

The uniform is a good first filter, along with some basic authentication (passphrase, awareness of common knowledge within that army (what do you mean you don’t know who Lt-Col KilledANaziWhilePeeing is‽), etc.). But beyond that, the army is organized in layers of teams.

The leaders of each team know the members of their teams (at the lower level) or the leaders of the team below them along with the leader above them and where each of these is supposed to be.

When you get a soldier that does not belong below you, you report them to the leader above you until. If they belong to a team below you, just send them to the leader responsible for them. So on and so forth until they can reach their team again.

If someone tries to infiltrate, you will end up in front of the leader of the team you were supposed to belong to, which will be able to certify you are not who you say you are. At which point, the infiltrator becomes either dead or a POW.

There is no need for big technological means to implement that and it scales rather well.