The Third Front Movement or Third Front Construction was a Chinese government campaign to develop industrial and military facilities in the country's interior. The campaign was motivated by strategic depth concerns that China's existing industrial and military infrastructure would be vulnerable in the event of invasion by the Soviet Union or air raids by the United States. The largest development campaign of Mao-era China, it involved massive investment in national defense, technology, basic industries, transportation and other infrastructure investments and was carried out primarily in secret.