What is illegal or highly suspicious about an order to deploy overseas? It's not like they're asking him to nuke a village or something.
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He has reason to believe those orders are coming from an illegal POTUS.
Does he have reason to believe his commanding officer, that person's commanding officer, the Chief of Staff of the Army and the Chairman of the Joint Chief's of Staff are illegal? If not then failure to obey their orders is a court martial offense.