Final answer:
Regions with significant instability and existing nuclear materials, such as states arising from the dissolution of the Soviet Union and countries in Asia and the Middle East, are considered the most likely sources for terrorists to obtain a "loose nuke." International treaties like NPT and START attempt to control nuclear proliferation, but illegal arms trades and non-state terrorist groups pose ongoing threats.
Step-by-step explanation:
The question of what region of the world is considered by experts to be the likeliest source for terrorists to obtain a "loose nuke" connects directly to concerns regarding the security of nuclear materials in regions with prevalent instability. After the collapse of the former Soviet Union, states such as Ukraine, Kazakhstan, and Belarus underwent periods of significant transition and instability, raising fears that non-state actors or terrorist groups could acquire nuclear weapons or materials. In addition to the remnants of the Soviet arsenal, regions with ongoing conflicts and unstable regimes, such as those in parts of Asia and the Middle East, notably North Korea and Pakistan, are also considered potential sources for terrorists to obtain nuclear materials. Despite international efforts like the Nuclear Non-Proliferation Treaty (NPT) and the Strategic Arms Reduction Treaty (START), the challenge remains due to the existence of illegal arms trades and the ability of non-state groups like al-Qaeda and ISIS to operate transnationally.