Why didn't the European nations attempt to save Constantinople from the Ottomans, while they sent thousands of soldiers to protect Vienna, a smaller and arguably less important city?

 The fall of Constantinople to the Ottoman Empire in 1453 marked the end of the Byzantine Empire and was a pivotal moment in history. The reasons why European nations did not intervene to save Constantinople while responding to the Ottoman threat at Vienna in later years are complex and involve geopolitical, religious, and strategic considerations. Here are some key factors:

1. **Distance and Communication:**

   - Constantinople was geographically distant from Western Europe, making communication and coordination challenging during that time. The speed of information and military response was limited by the technology and infrastructure of the era.

2. **Geopolitical Context:**

   - At the time of the fall of Constantinople, Europe was divided by internal conflicts, and many states were engaged in wars against each other. The political fragmentation and rivalries among European powers hindered the formation of a united response to the Ottoman threat.

3. **Religious Differences:**

   - Constantinople was a predominantly Christian city, but it followed the Eastern Orthodox branch of Christianity. Western Europe, particularly after the Great Schism of 1054, was predominantly Catholic. The religious divide contributed to a lack of unity in responding to the Ottoman threat.

4. **Focus on Other Priorities:**

   - European nations had their own internal and external challenges during the 15th century. Some were dealing with internal conflicts or dynastic struggles, while others were engaged in wars with neighboring states. The fragmented nature of Europe at the time meant that various regions had different priorities.

5. **Strategic Importance of Vienna:**

   - Vienna, in contrast to Constantinople, was strategically located in Central Europe. It was seen as a gateway between East and West and had significant symbolic and strategic importance. The Ottoman expansion towards Vienna threatened to advance deeper into Europe, making it a focal point for resistance.

6. **Changing Military Technologies:**

   - By the time the Ottomans reached Vienna in 1529 (a few decades after the fall of Constantinople), military technologies and tactics had evolved. The Ottoman threat to Vienna prompted a coordinated response from various European powers, recognizing the strategic significance of halting Ottoman expansion into Central Europe.

7. **Habsburg-Ottoman Rivalry:**

   - The Habsburg monarchy, particularly under rulers like Charles V, had direct territorial confrontations and rivalries with the Ottoman Empire. The Habsburg-Ottoman conflicts influenced the response to the Ottoman threat in Central Europe.

While the fall of Constantinople marked a significant historical event, the geopolitical context and challenges of the time contributed to the differing responses to the Ottoman threat in various regions of Europe. The defense of Vienna later became a focal point for resisting Ottoman expansion into Central Europe.