Complex sentence: Sarit justified his authoritarianism in two ways: firstly he argued for a return to Thai traditions of social law and order, and secondly he accelerated economic development under an economic plan designed by the United States and the World Bank that promoted market competition and private investment.
Split into simpler sentences: Sarit justified his action by arguing for a return to Thai traditions of social law and order. Sarit accelerated his country's economic development under a plan designed by the United States and the World Bank that promoted both market competition and private investment.