Panama was inhabited by indigenous tribes before Spanish colonists arrived in the 16th century. It broke away from Spain in 1821 and joined the Republic of Gran Colombia, a union of Nueva Granada, Ecuador, and Venezuela. After Gran Colombia dissolved in 1831, Panama and Nueva Granada eventually became the Republic of Colombia. With the backing of the United States, Panama seceded from Colombia in 1903, allowing the construction of the Panama Canal to be completed by the US Army Corps of Engineers between 1904 and 1914. The 1977 Torrijos–Carter Treaties agreed to transfer the canal from the United States to Panama on December 31, 1999. The surrounding territory was first returned in 1979.