Which American President has done the most damage to US national security?

Obvious choice GW Bush.

Clinton:
WASHINGTON, May 10— The Clinton Administration notified Congress today that it had approved the export of technology to China to permit the launching of a communications satellite aboard a Chinese rocket next month.
President Clinton said in a letter to Congress that the transfer would not harm national security or significantly improve China's military capability in space. The President was required under a 1998 law to certify that all such technology exports are in the national interestThe certification was the first such notice to Congress under the law, which was passed in the aftermath of a Congressional uproar last year over the transfer of sensitive missile technology to China.

Mr. Clinton's notification also follows by less than a week the release of a report by the Senate Select Committee on Intelligence, which concluded that lax monitoring of the launching of American-made satellites aboard Chinese rockets had enhanced the accuracy of China's ballistic missile arsenal.
http://www.nytimes.com/1999/05/11/world/clinton-approves-technology-transfer-to-china.html

I would say Franklin Pierce and James Buchanan are tied, but I am a bit of a history buff.

Simple. Obama

Ah, Jefferson Davis wasn't a US President. I think that Clinton and Carter did more damage to national security. Clinton allowed OBL to traipse around the world creating global chaos with al Qaeda while Clinton dismantled the US Military. Carter got bullied by the Soviet Union and Iran, both of which we are still paying for. Obama precipitously withdrew from Iraq and set the conditions for Benghazi and ISIS but both pale in effect to the damage that Clinton and Carter did.

Well that's is interesting. He was never the President of the United States. He was the President of the Confederate States .

The current inhabitant of 1600 Pennsylvania Ave has done more to damage and destroy the United States Military than the combined forces of the Axis Powers prior to 1943.

The worst days in US Military History: The Attack on Pearl Harbor, Kasarene Pass, Task Force Smith, and the inauguration of Barak Obama.

That is the most inane response possible to this question. Completely ignoring the fact that a draft dodger/deserter led us into an unwinnable war over a known lie for the purpose of enriching his family and his cronies, while getting over 4400 of our brothers and sisters at arms killed, and more than 100,000 permanently disabled. All this for nothing more than war profiteering and ultimately destabilizing the Mideast and western Asia with what we have today. For an obviously educated man, you are incredibly, willfully obtuse.

Our current President has done more infrastructure damage to the United States than any other President, and might I add ... more damage than Adolf Hitler!

To each generation ... :)