Final answer:
The effect of World War II and the three-world order on the modern state affirmed the nation-state as the primary organizing structure for communities and economic development, despite superpower competition and global ideological tensions.
Step-by-step explanation:
The best description of the effect of World War II and the subsequent three-world order on the modern state is that across all three blocs, the nation-state was affirmed as the primary form for organizing communities and promoting economic development. This is due to the fact that subsequent to World War II, despite the emergence of the Cold War and diversified alliances, the concept of nation-states remained a key organizational structure.
After World War II, the United States and the Soviet Union came out as superpowers and influenced global politics significantly. Their competition during the Cold War saw both sides vying for influence and control over resources in various third world countries emerging from colonial rule or undergoing reforms. With the collapse of the Soviet Union, the bipolar global order diminished, leading to greater complexity in international relations and the resurgence of nationalism in many regions.
Despite shifts towards liberalization or authoritarianism, the construct of the nation-state persisted, further strengthened by nationalist movements and the need for post-war economic development. The emergence of empires as a viable form of community organization declined significantly after WWII, whereas nation-states continued to be the fundamental unit of political identity and sovereignty.