Does international law treaty law diminish a nation's sovereignty?

This is a surprisingly deep question; it cuts to the core of what we mean by "sovereignty", and strongly overlaps with similar deep questions about personal autonomy.

Which nation is more free: The one that never has to cooperate with other nations, or the one that is free to enforce cooperation with other nations?

Which person is more free: The one who never has to obey any contract, or the one who is free to make contracts with others?

A great deal of ink has been spilled on this question.

My perspective on this matter comes from being a cognitive economist, and that pushes me pretty strongly toward saying that the right to contract is the form of autonomy worth protecting. Human beings often find themselves in coordination problems, where what I want to do depends strongly on what I expect you to do---and vice versa. By establishing contracts, we can coordinate our actions and solve such problems, achieving a better outcome for everyone. (Such conflicts can even occur within a person---the conflict between the me now that wants to eat the cookie and the me later that doesn't want to have eaten so much sugar. We often make "deals" with ourselves to resolve these conflicts---"Okay, just one cookie today, but none tomorrow." Are such deals enforceable? Your mileage may vary.)

There are some extreme examples where we might want to limit the enforceability of contracts---for example, we don't allow people to sell themselves into slavery, because that would mean giving people the "freedom" to give up all their future freedom. But for the most part, personal autonomy is maximized by letting people make and enforce contracts.

And along similar lines, if national sovereignty is to be worthwhile, it must be in a form that allows nations to make contracts with one another. International cooperation is vital to the success of all nations---especially in an age of intercontinental nuclear weapons. It would impose more on the freedom of a nation's people to advance their common interest to say that they can never make binding agreements with other nations.

National sovereignty also poses another complication, which is that nations are not natural entities. Human individuals exist on their own, and while we do depend on one another, each of us makes our decisions as a meaningfully autonomous agent. But nations are not like this; they are social constructions that may be built out of the interests of millions of people. It is actually quite possible for a nation to not have any coherent desires or interests--because there is so much conflict within it that no single desire can attain a clear majority. In a case where that happens, it's honestly not clear what national sovereignty means at all.