It is certainly possible to argue that international relations weaken states. To argue this, we would note that international institutions such as the United Nations create international laws that limit what states can do. For example, the World Trade Organization issues rules that outline the sorts of subsidies that states can and cannot give domestic companies. This weakens the state by reducing the degree to which it is sovereign.
However, it is also possible to say that such institutions do not weaken the state. We can say that states enter into these institutions voluntarily because they feel that giving up some of their sovereignty will be beneficial to them in the long run. They feel that such institutions will make them safer and more prosperous since they set up rules that make the international order less anarchic.