Learn French: L'assaut aura raté hier. - The assault must have failed yesterday.

This is the epistemic future (futur de probabilité) that you are currently studying. In French, you can use the futur antérieur (future perfect) to express a strong assumption or probability about something that has already happened. Even though it is formed like a future tense, in this context it shows you are making a deduction about a past event, which is why it pairs perfectly well with a past time marker like hier.

Why is it translated as 'must have failed' instead of 'will have failed'?

Because expressing a past deduction in English usually relies on 'must have'. If you translate it literally as 'will have failed yesterday', it creates a confusing time-travel paradox in English. When the French futur antérieur is used to express probability, 'must have' plus the past participle is almost always the most natural translation.

I usually see rater used for missing a bus or a train. How does it mean 'failed' here?

The verb rater broadly means 'to miss the mark'. When applied to transport or an appointment, it means 'to miss' (like rater le train). But when applied to an action, a plan, or a test (like l'assaut), it translates to 'to fail' or 'to go wrong'. Its opposite in this context would be réussir (to succeed).

Could I just say 'L'assaut a dû rater hier' to express the exact same thing?

Yes, absolutely! Using the verb devoir in the passé composé (a dû rater) is another very common way to say 'must have failed'. The version using the futur antérieur (aura raté) feels a bit more formal or literary, often showing up in news reports or historical deductions, whereas using devoir is the standard for everyday conversation.