The militaristic, anti-North-Korean content of US President Donald Trump’s first speech at the United Nations may have surprised and even shocked many, but did it come out of nowhere? In fact, it comes from two places: a long-term decline in US internationalism and a more recent, under-appreciated deformation of US foreign policy doctrine on war. Comparing statements by the first two US presidents of the UN era with statements by Mr. Trump’s two immediate predecessors illustrates both points.

The comparison is important, the trend is clear, and the conclusion is inescapable. First, US internationalism has declined precipitously from its post-World-War-Two zenith under Presidents Harry Truman and Dwight Eisenhower. Second, anyone dissatisfied with the jingoistic tone and content found in Mr. Trump’s speech ought to hold Presidents George W. Bush and Barack Obama at least partially responsible as the forerunners and enablers of Mr. Trump. Mr. Bush changed the long-established, restrained content and tone of American foreign policy concerning preventive war, and Mr. Obama did nothing to restore the collegiality of Mr. Truman or the restraint of Mr. Eisenhower to the American international outlook in this regard.

Consider US internationalism at it’s height under Mr. Truman and Mr. Eisenhower. Mr. Trump’s speech quoted Mr. Truman’s message to Congress concerning the Marshall Plan to rebuild Europe after World War II. The quotation used in Mr. Trump’s speech emphasized US commitment to principles of the UN Charter and the necessity for member states to be independent and capable. However, in a part of the message not mentioned by Mr. Trump, Mr. Truman quoted the preamble of the UN Charter to highlight American commitment to the UN’s ultimate mission “to save succeeding generations from the scourge of war.”

Mr. Eisenhower, Mr. Truman’s successor, further specified US policy in a press conference in which he took preventive war off the table as a potential tool of US Cold War diplomacy.

This renunciation of preventive war remained settled, bipartisan US policy until the post-9/11 Bush Doctrine by which Mr. Bush disfigured US internationalism by undoing the policy of Mr. Eisenhower.

Mr. Bush, although he used the language of preemption, returned preventive war to the list of options the United States would consider when he said to cadets graduating from West Point, “If we wait for threats to fully materialize, we will have waited too long.”

Mr. Obama perpetuated the shift away from Mr. Eisenhower’s policy. Although some debate the existence of a particular Obama Doctrine, Mr. Obama himself said, “The doctrine is: We will engage, but we preserve all our capabilities.” This preservation would seem to include the ability to wage a preventive war. Certainly, Mr. Obama never explicitly retracted this aspect of the Bush Doctrine.

As a result, preventive war became a settled, bipartisan US foreign policy option over the course of a two-term Republican presidency followed by a two-term Democratic presidency.

Intended or not, recognized or not, the face of US internationalism had changed. If the Truman-Eisenhower era marked a rise, then the Bush-Obama era marked a fall.

This fall could have grim implications for the world as a whole and the Asia-Pacific region in particular.

Mr. Trump, the inheritor of the Bush-Obama policy, told the UN General Assembly, “The United States…if it is forced to defend itself or its allies…will have no choice but to totally destroy North Korea…The United States is ready, willing and able…” Words like these underscore the warping of American foreign policy doctrine that had been ongoing yet underappreciated.

Will the Bush-Obama permissiveness regarding preventive war prove to be the greatest foreign policy mistake of these two presidencies? I do not know that.

I do know one thing, however: Mr. Trump has taken the post-9/11, bipartisan US doctrine on preventive war to its logical extreme, and the world has gasped.