Gateway Arch in St. Louis, Missouri. Photo by Kathy Weiser-Alexander. The St. Louis Gateway Arch was erected to commemorate the Louisiana Purchase of 1803 and the city's role in the rapid westward expansion that followed. In the late 1940's, architect Eero Saarinen won a competition to design a monument honoring the spirit of western migration, however he would die in 1961 before seeing the construction of the Arch that was completed on October 28, 1965.