The King is always right, always legal?

The torture memos--Justice Department higher-ups saying it was OK to torture, are depraved. After 9-11, the “gloves came off,” asserted Vice President Cheney , and screws were tightened in prison cells from Guantanamo to Bagram. The torturers and their enablers need to be punished but what can stop a President from instigating these horrid abuses again? George Bush simply stated that whatever he did was right and legal. And neither Congress, the Judiciary or the Press were able to resist. Over the years the US presidency has become omnipotent, especially so when it comes to foreign policy. This tendency towards monarchy needs to be checked, but what is the mechanism?