Recent decades have witnessed a dramatic shift in the cross-border collaboration mode of researchers, with countries increasingly cooperating and competing with one another. It is crucial for leaders in academia and policy to understand the full extent of international research collaboration, their country's position within it, and its evolution over time. However, evidence for such world-scale dynamism is still scarce. This paper provides unique evidence of how international collaboration clusters have formed and evolved over the past 50 years across various scientific publications, using data from OpenAlex, a large-scale Open Bibliometrics platform launched in 2022. We first examine how the global presence of top-tier countries has changed in 15 natural science disciplines over time, as measured by publication volumes and international collaboration rates. Notably, we observe that the US and China have been rapidly moving closer together for decades but began moving apart after 2019. We then perform a hierarchical clustering to analyse and visualise the international collaboration clusters for each discipline and period. Finally, we provide quantitative evidence of a `Shrinking World' of research collaboration at a global scale over the past half-century. Our results provide valuable insights into the big picture of past, present and future international collaboration.