That would be U.S. President Richard Milhous Nixon.
The technicality is in the title within your question. The US President is the civilian "commander in Chief" of the US Armed Forces as stated in the US Constitution, however, during NATO supported conflicts, in the past, there has been a military position of Supreme Commander of NATO forces, which reports to the US President and NATO.
Wanted to expand NATO to increase security and military influence.
Bill Clinton
The US joined NATO, because after WWII, President Truman needed to keep up the subterfuge that they needed an alliance, because the US citizens didn't know about the formation of the atomic bomb.
He became President of Columbia, first head of NATO and President of the US.
No, he changed his mind about this and now NATO is fine with him.
He is an American Army General who ran as a Democrat for US President in 2004. He also commanded NATO in Europe.
When Truman was president.
NATO does not have a president. It's acting head is the Secretary General. The current Secretary General is Anders Fogh Rasmussen, former prime minister of Denmark.
CONFIDENTIAL FGI NATO
No he could not, and in fact, no US president could. NATO is an organization comprised of twenty-eight member countries. One country might decide to withdraw from NATO, but the organization would continue to exist. Thus, even if the US under Mr. Trump left NATO, there would still be many other nations that would continue to participate. As the NATO website says, "NATO’s essential purpose is to safeguard the freedom and security of its members through political and military means." Of course, the US is considered a very important part of NATO, but with or without its participation, other nations would still be members.
The USSR created their WARSAW Pact to counter NATO.