Are all soldiers considered tyrants in systems that do not rule by Allah's law, or are there exceptions to that, and what are they?

The texts of Islamic law indicate the disbelief of whoever rules by other than what Allah has revealed. This falls under the concept of Taghut (tyranny/idolatry), which Ibn al-Qayyim mentioned as having among its chief forms, whoever rules by other than what Allah has revealed. There is a detailed explanation regarding the ruling on whoever rules by other than what Allah has revealed, distinguishing between major and minor disbelief, which varies according to circumstances. As for the soldier, he is not considered to have committed a forbidden act merely by working as a soldier, nor is he considered a Taghut. If his work does not result in an evil deed or aid to an oppressor, then he is not sinful. Moreover, there might be an overriding benefit in joining this work despite the presence of some harms.