1878. Setting up a de jure autonomous Bulgarian principality. Third Bulgarian state.

The Treaty of San Stefano was signed on 3 March 1878, setting up a de jure autonomous Bulgarian principality on the territories of the Second Bulgarian Empire. The Great Powers immediately rejected the treaty out of fear that such a large country in the Balkans might threaten their interests. The subsequent Treaty of Berlin provided for a much smaller state comprising Moesia and the region of Sofia, leaving large populations of Bulgarians outside the new country. This played a significant role in forming Bulgaria's militaristic approach to foreign affairs during the first half of the 20th century. The Bulgarian principality won a war against Serbia and incorporated the semi-autonomous Ottoman territory of Eastern Rumelia in 1885, proclaiming itself an independent state on 22 September 1908.

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1983. The town regained its independence.

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1942. Le Pecq was bombed by mistake.

The Royal Air Force bombed the Renault factory in Boulogne-Billancourt on the night of 3 to 4 March, following an error in aviation, they bomb the city of Pecq causing the loss of 47 civilians.

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