The Arab Spring is one of the main triggers of the contemporary Middle East crisis. Indirectly, it strengthened the radical wing of the Islamist movement, led to the wars in Syria and Yemen, intensified regional competition, and destabilised the region. However, this regional instability is not a recent phenomenon. It has its roots in the aftermath of the First World War when the combination of internal and external factors transformed this previously tranquil region into a high-risk zone. The main ideological force in the Middle East in the 1950s, Arab nationalism, ceased to play an important role in the contemporary inter-Arab relations. Still, it would be a mistake not to consider it at least partially responsible for the present-day situation in the region. The article focusses on the developmental dynamics of Arab nationalism. That entails the analysis of the circumstances under which its liberal, pro-European discourse has been gradually transformed into the authoritarian Pan-Arabism, of its impact on the political arrangement and relations in the region, as well as the analysis of the reasons that caused the idea of Arab political unification to cease functioning as a significant framework of individual and collective action. Lastly, the article discusses the political legacy of Arab nationalism. It withdrew from the political scene of the Middle East but has left behind the autocratic regimes determined to stay in power at all cost. These repressive regimes, permanently at “war” with their own citizens and with hegemonic aspirations towards neighbouring countries, are Arab nationalism’s political legacy that bears a significant responsibility for the violence that engulfed the region after the collapse of the Arab Spring and for the resulting Middle East nightmare.