Research the "Treaty of Tordesillas"... by which I mean, don't just look up a one paragraph summary, spend a bit of time with it. A few papal bulls at least. Ideally in a modern formulation you should encounter the term "Requerimiento" without having directly searched for it, and how the modern iteration of the line-of-demarcation doctrine is a reaction to that in a way the 1494 version didn't have to contend with.
I'm not doing advocacy here, I'm giving pointers on how to come to a deeper understanding of what is going on.
It is also important to understand that the United States is directly attacking this doctrine by just sitting there. You are free to agree with the United States. Again, I'm just giving something to guide people to a deeper understanding of what is going on beyond the surface level playground politics of it all. There's more going on than that.