In none of his writings are the opinions of Las Casas on questions of the rights of man and the functions of government more lucidly set forth, and while many of the arguments on which he rested his propositions, and which were consonant with the prevalent spirit of his times, would not secure universal assent in our day, there is not one of the essential principles of his thesis, that has not since been recognised as inherently and indisputably just.
"I propose in this article to demonstrate three propositions; first, that all the Indians who have been enslaved since the discovery of the New World, have been reduced to this sad condition without right or justice; second that the majority of Spaniards who hold Indian slaves do so in bad faith; and third, that this imputation is also applicable to such Spaniards as have not acquired their slaves by right of repartimiento but have obtained them from other Indians."
He combated the almost universally accepted theory that justifiable conquest conferred the right of enslaving the conquered, and he maintained that the most that might be exacted from a conquered people, even from those who had actively resisted, was recognition of the government established by the victorious party; taxes were justifiable and must be paid, and prisoners of war might be held until the close of hostilities, while extra burdens might be laid upon the country during the period of military occupation. Not one of these principles was at that time acted upon by any Christian power engaged in war with uncivilised nations, yet every one of them is now placed beyond dispute by the universally accepted principles of international law.
Wars unjustly undertaken, according to Las Casas, could confer no rights, because right is not founded upon injustice, and he defined war as unjust when undertaken without the sanction of legitimate authority, or even when ordered by legitimate authority, but without sufficient motive or provocation. This touched the question of the Indians very closely, for most of the Spanish invasions of the different islands and the countries of the mainland were begun without any authority from, or even the knowledge of the Spanish government. No Spanish sovereign ever authorised the invasion or conquest of any of the countries, on which their distant and self-styled representatives embarked, for motives of personal aggrandisement or in a pure spirit of adventure. Both Velasquez in Cuba and Cortes in Mexico were destitute of any royal authority for their undertakings, and only the splendour of their successes sufficed to condone their license, when they were able to confront the King with a profitable fait accompli. The royal instructions to all governors and representatives of the Spanish Crown were, on the contrary, filled with injunctions to treat the Indians humanely, to provide for their conversion and instruction by pacific means, and on no account to employ force save for self-defence.