If you think countries should have open borders, then Joseph Carens, a prominent political theorist, is on your side. But, why?
In the state of nature before society began setting boundaries for humanity into separate countries, everybody has equal moral worth and value.
In fact, isn’t restrictive citizenship almost the same as feudal birthright privileges? As such, Carens even goes as far as saying that political borders (i.e. countries, nations) are incompatible with the equal status of individuals!
The most fool-proof way to defend your argument is against people of all different perspectives and theories. This is exactly what Carens does. He shows that a utilitarian, Nozickian, and Rawlsian ALL lead to the same conclusion: countries should all have largely open borders!