Is this passive voice misuse? [duplicate]

What is wrong with this? I have quintupple checked it, but grammarly tells me that I have passive voice misuse. Can someone help?

"The Japanese were warned that one of their cities, Hiroshima, which is also where their 2nd army was, would vanish."

Solution 1:

Ignore it. This is the outcome of rule creep: gradually escalating sound syntactical advice into a senseless grammatical "rule".

Level 1: Overuse of the passive can definitely be a stylistic vice (yes, that's vice, not device).

Level 2: Overuse of the passive is frequent enough that many teachers and writers on style find it necessary to issue stern warnings against it.

Level 3: Insensitive writers on style have been known to elevate such warnings against overuse of the passive to deprecation of any use of the passive.

Level 4: Incompetent editors and grammar checkers mechanically mark every use of the passive as an 'error'.

Solution 2:

Use of the passive in English varies with writing style and field. Some publications' style sheets discourage use of the passive voice, while others encourage it. If you are discouraged from using the passive voice then you need to pay attention to grammar checkers such as Grammarly.

When looking at passive voice there is an example where

"Caesar was stabbed by Brutus" is in the passive voice. The subject, Caesar, indicates the person acted upon. The agent is expressed here with the phrase by Brutus, but this can be omitted. The equivalent sentence in the active voice is "Brutus stabbed Caesar", in which the subject denotes the doer, or agent, Brutus.

So the sentence you have is in the passive voice as the subject is the Japanese who were warned. To change to an active (non-passive) voice you need to add who gave the warning and rearrange the first part of the sentence. If it was the Chinese who warned the Japanese, you could say

The Chinese warned the Japanese that one of their cities.....