President Donald Trump speaks beside NATO Secretary General Jens Stoltenberg at the start of the NATO summit at their new headquarters in Brussels, Belgium, May 25, 2017.

President Donald Trump's failure to endorse the central principle of NATO — the members' commitment that they will defend each other militarily — shocked his national security advisors as much as it did America's allies, according to Politico.

Trump alarmed members of the 28-nation alliance late last month when he stopped short of endorsing Article 5, the clause in the NATO charter that requires allies to automatically come to each other's defense if an attack happens. That provision has been used only once — when Europe and Canada came to America's defense in Afghanistan after the Sept. 11, 2001, attacks.

National security advisor H.R. McMaster, Defense Secretary James Mattis and Secretary of State Rex Tillerson worked to include an Article 5 endorsement in the speech and expected it to be there, Politico reported Monday, citing five sources familiar with the events.

The senior advisors did not know it had been removed from Trump's remarks until he actually spoke, according to the report.

It is not clear if Trump made the decision himself to remove an Article 5 endorsement, or if White House advisors outside of the foreign policy team yanked the reference from the speech.

The White House did not immediately respond to a request to comment on this article.

Read the full Politico report here.