Etydhv

I think there's a word or an adjective for it, but I don't remember what it was. I don't want to say plan B, because that's too informal, but I remember there was a good word for it. I just don't remember it anymore.

For example:

The army was flanked by the enemies unexpectedly so that the general
had to rely on his ___ plan.

That's most often called a backup plan:

He was used as a backup plan when the remaining members of the gang failed to accomplish their mission.

This usage is also mentioned in Merriam-Webster's definition for backup:

1 a: one that serves as a substitute or support

// I brought an extra pencil for backup.

// a backup plan

The Oxford Dictionary has

fallback

NOUN

1 An alternative plan that may be used in an emergency.

Make sure you have a fallback plan in case something goes terribly wrong

So the sentence can be

The army was flanked by the enemies unexpectedly so that the general had to rely on his fallback plan.

In the context, this is particularly apt.

That could also be called a contingency or a contingency plan. The Oxford Living Dictionaries gives sense 1.1 under noun as :

A provision for a possible event or circumstance.

"stores were kept as a contingency against a blockade"

The process of developing one or several alternate plans is often called contingency planning.

Dictionary.com gives as an example:

Turkey has cooperated at times with Israel and the West on contingency planning for Syria during its civil war. (Israel Bombs Gaza While Hamas’ Kidnapping Mastermind Sits in Turkey|Eli Lake|July 1, 2014|DAILY BEAST)