The letter says Congress and the American people would hold Trump responsible if a "careless or ill-advised miscalculation" results in conflict that endangers US military members or allies in the region.
"I think that the president, again, as commander-in-chief, I think he felt it necessary to issue a very strong statement directly to North Korea", he added.
Trump also says the nation's nuclear arsenal is in "in tip-top shape" and getting stronger.
The Democrats noted that the administrations of Presidents Barack Obama, George W. Bush and Bill Clinton each considered military action against North Korea but ruled it out because it would have run the risk of a counter-response from Pyongyang that could threaten South Korea and the more than 30,000 USA service members and 100,000 US citizens living there.
Lisa Monaco, the former homeland security adviser to President Barack ObamaBarack ObamaDemocrats introduce another "false hope" act to immigrants IL makes "Barack Obama Day" a state holiday GOP senator: I wish Republicans had stood up to birtherism MORE, warned President Trump on Wednesday against making improvised or uncoordinated statements on North Korea, saying the situation offers "no room for error".
Catherine Ray, a spokeswoman for the European Union's executive arm in Brussels, told a news briefing on Wednesday the situation in North Korea is of "great concern" and called on the North to "refrain from any further provocative action".
North Korea said it was "carefully examining" a plan to strike Guam, which is home to about 163,000 people and a US military base that includes a submarine squadron, an airbase and a Coast Guard group.
But there is "more continuity than there is change" during the review, Kimball said, and no official guidance is offered until after the review is complete.
If Ukraine and Russian Federation can be said to engage in public diplomacy on Twitter when they battle each other with memes, then it could also arguably be a form of diplomacy for Trump to seek to deter North Korea with tweets, alarming as those tweets may be. "You've got to be sure you can do what you say you're going to do", he said in a radio interview.
Sebastian Gorka, deputy assistant to Trump, interpreted the president's latest tweet as a warning to Pyongyang.
Political analysts have been warning that the White House should take into consideration China's position on this boiling issue.
She noted more than 200,000 Americans lived in the region and that 26 million people reside in Seoul, the South Korean capital.