Which major international issue was highlighted by concerns over Iran's nuclear capabilities?

The primary concern regarding Iran's nuclear capabilities centers around nuclear proliferation, which is the spread of nuclear weapons and related technology to states that do not already possess them. The international community, particularly countries involved in negotiations and treaties like the Treaty on the Non-Proliferation of Nuclear Weapons (NPT), has been alarmed by Iran's advancements in nuclear technology, fearing that these capabilities could lead to the development of nuclear weapons. This raises significant security issues for not just the region but the world as a whole, as the potential for nuclear weapons to fall into the hands of unstable governments or non-state actors poses a great risk.

The focus on nuclear proliferation is crucial because it encompasses the broader implications of states acquiring nuclear capabilities and the potential arms race that could ensue. Addressing this issue has led to numerous diplomatic efforts, including sanctions and negotiations, aimed at preventing Iran from developing nuclear weapons, making nuclear proliferation the most pressing major international issue related to Iran's nuclear capabilities.