Mont Blanc
Ownership of the summit

At the scale of the Mont Blanc massif, the border between Italy and France passes along most of the main Alpine watershed, from the Aiguille des Glaciers to Mont Dolent, where it reaches the border with Switzerland. However, its precise location near the summits of Mont Blanc and nearby Dôme du Goûter is disputed. Italian officials claim the border follows the watershed, splitting both summits between Italy and France. In contrast, French officials claim the border avoids the two summits, placing them entirely with France. The size of these two (distinct) disputed areas is approximately 65 hectares (160 acres) on Mont Blanc and ten hectares (25 acres) on Dôme du Goûter.

In 1723, the Duke of Savoy, Victor Amadeus II, acquired the Kingdom of Sardinia. The resulting state of Sardinia was to become preeminent in the Italian unification.

Citing the Treaty of Paris (1796), France claims that the historical border of Savoy and Piedmont diverged from the watershed line, fully encompassing the summit of Mont Blanc within Savoy and consequently, within French territory.

After the Napoleonic Wars, the Congress of Vienna restored the King of Sardinia in Savoy, Nice, and Piedmont, his traditional territories, overruling the 1796 Treaty of Paris. Forty-five years later, after the Second Italian War of Independence, it was replaced by a new legal act. This act was signed in Turin on 24 March 1860 by Napoleon III and Victor Emmanuel II of Savoy, and deals with the annexation of Savoy (following the French neutrality for the plebiscites held in Tuscany, Modena, Parma and Romagna to join the Kingdom of Sardinia, against the Pope's will). A demarcation agreement, signed on 7 March 1861, defined the new border. With the formation of Italy, for the first time, Mont Blanc was located on the border of France and Italy, along the old border between the department of Savoy and that of Piedmont, formerly belonging to the Savoyard state.

The 1860 act is still legally valid for both the French and Italian governments. Italy claims, that the border was moved by France in 1865, when surveys carried out by a cartographer of the French army, Captain JJ Mieulet, incorporated the summit into French territory, making the state border deviate from the watershed line, and giving rise to the differences with the maps published in Italy in the same period.

Modern Swiss mapping, published by the Federal Office of Topography, plots a region of disputed territory (statut de territoire contesté) around the summits of both Mont Blanc and the Dôme du Goûter. One of its interpretations of the French-Italian border places both summits straddling a line running directly along the geographic ridgeline (watershed) between France and Italy, thus sharing their summits equally between both states. However, a second interpretation places both summits, as well as that of Mont Blanc de Courmayeur (although much less clearly in the latter case), solely within France.

NATO maps take data from the Italian national mapping agency, the Istituto Geografico Militare, which is based upon past treaties in force.
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