Question

the army chief,with his men__was__killed. correct or not ​

Answer 1

This statement is totally correct.

Just to confuse you, they included the men too, so you think that "were" should be used here. But see the sentence clearly, they are talking about the army chief which is a singular person. So we will use "was" here instead of "were".

If the sentence was "Two people, with the army chief was killed", then this sentence would be incorrect. Because we are talking about two persons and that's plural. Then we would use "were" instead of "was".

Hope it helps! :)