A crime is a violation of a contract, where laws are properties of a contract for norms within a society fo people with similar goals, manners, ethics, morals, language and reproductive strategies. Even if the contract is nautural law, and even if natural law only applies to people within a government, not across governments. Human rights are a post-war extension of natural rights. We have attempted to legitimize conquest only in those cases where those extended rights are violated. While it is arguable that natural rights existed at the time, it is also arguable that human rights did not exist at the time. But since none of these contracts apply, conquest is not a crime. There was no contract. The reason for conquest, whether by violence, or immigration, or import of religion, or revolution, is to replace one set of rights and obligations with another set of rights and obligations.
You may dislike conquest. You may argue in retrospect that we should not conquer primitive peoples. You may argue that we should not conquer primitive peoples even if we are more beneficial conquerors than any of our competing conquerors, and therefore commit the lesser of evils.
Conquest is not a crime unless immigration, new religion, new political parties, are a crime. Immigration, religion and political parties are implemented under the threat of violence, and therefore the only difference is the rate and means by which one conducts conquest, and the rapidity at which rights, obligations and the allocation of property is rearranged.
It is not clear that the french revolution, and its bloody excesses, nor the philosophy that it created, which led to marxism, and 100M murderous deaths because of marxism was not a conquest. It was. And nothing good came of it.
Conversely, it is quite clear that the conquest spread by anglo-empirical-science, acounting, property rights and capitalism were a conquest, that in turn, raised billions out of mysticism, ignorance and poverty.