Well, conquer Texas and implement that. It'll be your right to do so. As it is, the US government holds ownership of Texas and so they get to decide the rules for coming in.
Like talking to a wall.
Of course they have the power.
I am only talking about what is right, not what I am planning to do tomorrow.
No, that's not right. We don't want a bunch of Americans just wandering around up here all willy-nilly. You're violent and you have a lot of guns, so border control is necessary. It's the same thing for the US. They should have the ability to screen travellers and control who is able to come into their country.
If I want to go to Ireland and visit my ancestral homeland, I'm quite able to do so but I need to stop at the border and let the Irish authorities know that I'm there. If they have a valid reason to stop me at the border, they have the ability and authority to do so. Similarly, if a Mexican wants to go and visit hi anccestral homeland in Texas, he has the ability to do so, but he needs to stop at the border and let the US authorities know that he's there and if they have a valid reason to stop him, they should have the ability and authority to do it before someone they don't want in their territory enters their territory.