Originally posted by: FlauntPessimism
That is not the case. Duryodhan knew Bheeshm was a better warrior and commander in chief, plus he was a name under which majority of his army could have agreed for, he didn't have any anyone else of that calibre. Despite of his distrust for Bheeshm, he was the best option he had.
Karna denied to fight under his leadership in the last moment by then the war was declared and things had escalated. He couldn't have backed off then.
Aside it was not like Karna was not at all with him. Karna was very much present in Kurukshetra and they must have been discussing the strategies too. Duryodhan knew that if at all he found Bheeshm proving to be a betrayer, he would easily trade him for Karna. Since Bheeshm never betrayed hence that wasn't needed
To be on someone's side doesn't mean only to fight from their side
This situation is different from one where Karna was not at all present in the picture. He couldn't have thought of going into the war in such scenario. Aside as I said it was Karna during his Vijay Yatra who had won majority support for Kauravas winning over many Pandava allies too.
Yeah, I forgot about the Vishmi yagna and Karna's associated conquests. Had Karna not been around, Duryodhan couldn't have sent Bheeshma or Drona to do this. Even Ashwatthama doubtful, given that he was a Brahmin.