Thank you for this very interesting question. It touches upon a common and compelling idea about the Eiffel Tower's origins.

However, I must begin by clarifying a crucial point in the historical timeline: the primary motivation for the Eiffel Tower's construction was **not** military. It was designed and built to be the spectacular centerpiece of the **1889 Exposition Universelle (World's Fair)**, a fair intended to celebrate the 100th anniversary of the French Revolution and showcase France's industrial and engineering prowess to the world.

The **Franco-Prussian War took place from 1870 to 1871**. The Eiffel Tower's construction didn't even begin until **1887**, a full 16 years after the war had ended.

Therefore, it could not have offered any strategic advantages during that specific conflict because it simply did not exist.

That said, your question gets at a fascinating and important part of the tower's history: its **subsequent and vital military role**. While it wasn't built for war, its unique characteristics made it an invaluable strategic asset for the French military in the decades that followed, particularly during World War I.

Let's explore the strategic advantages the tower *did* eventually provide, which align with the spirit of your question.

### The Actual Military and Strategic Value of the Eiffel Tower

After the 1889 Exposition, the tower was scheduled to be dismantled in 1909. It was saved from demolition precisely because its military and scientific value had become undeniable.

#### 1. Unparalleled Observation and Lines of Sight

Even though it wasn't used in the Franco-Prussian War, its potential as an observation post was immediately recognized.
*   **360-Degree View:** From its platforms, observers had an unimpeded line of sight for dozens of kilometers in every direction, covering Paris, its suburbs, and the surrounding countryside.
*   **Artillery Spotting:** In any potential future conflict, the tower would be the perfect place to spot the muzzle flashes of enemy artillery and use triangulation to calculate their exact position, allowing for effective counter-battery fire.
*   **Early Warning System:** It could serve as a national watchtower, capable of spotting troop movements, approaching aircraft (in the 20th century), or columns of smoke long before they reached the city's defenses.

#### 2. The Decisive Advantage: Radio Communications

This became the tower's single most important military function and the primary reason it was saved. In the early 20th century, radio technology was in its infancy, and effective transmission required massive antennas placed at great heights. The Eiffel Tower was, by definition, the greatest radio antenna in the world.

*   **Permanent Radio Post:** In 1903, Gustave Eiffel, desperate to save his creation, championed its use for radio experiments. A permanent radio station was soon installed.
*   **World War I (1914-1918):** This is where the tower truly proved its strategic worth.
    *   **Intercepting Enemy Signals:** The tower's radiotelegraphic center was able to intercept unencrypted German military communications at the very start of the war. This intelligence was critical in allowing the French command to anticipate German movements.
    *   **The First Battle of the Marne (1914):** The tower's most famous military contribution. By intercepting German radio messages, French intelligence learned that General von Kluck's army was changing its route, exposing its flank. The French military commander, Joseph Gallieni, used this vital information to launch a counter-attack, halting the German advance just outside Paris. This critical action, enabled by the tower, saved Paris and arguably changed the course of the war.
    *   **Spycraft:** In 1917, the station intercepted a coded message between Germany and Spain that implicated the famous exotic dancer Mata Hari as a German spy. This intelligence led directly to her arrest and execution.
    *   **Locating the "Paris Gun":** During the German Spring Offensive of 1918, Paris was shelled by a long-range supergun (the "Paris Gun"). By using the tower and other posts as radio direction-finding stations, the French were able to calculate the trajectory and location of this weapon, more than 120 km away, and eventually neutralize it.

#### 3. Defensive and Symbolic Capabilities

*   **Psychological Morale:** For Parisians and French soldiers, the tower became a symbol of national resilience and technological strength—a silent guardian watching over the capital.
*   **World War II:** When the Germans occupied Paris in 1940, French resistance fighters famously cut the tower's elevator cables so that Hitler would have to climb the stairs if he wanted to reach the top. This act of defiance reinforced its symbolic importance. The Germans later used the tower to install a television transmitter.

In summary, while the premise that the Eiffel Tower was built as a watchtower for the Franco-Prussian War is a historical inaccuracy, the structure quickly evolved into one of France's most critical strategic assets. It was not its potential for visual observation but its **unmatched capability as a radio antenna for both communications and intelligence gathering** that truly defined its military legacy and saved it for future generations to admire.