It sounded like the dullest of developments: the award of a big Pentagon I.T. contract was postponed so the new defense secretary could review it.

[Update: Microsoft wins Pentagon’s $10 billion JEDI contract, thwarting Amazon.]

But the competition over the $10 billion, 10-year contract to transform the military’s computing systems has been fierce, and the highly unusual, last-minute intervention by President Trump this week was another example of his willingness to shatter Washington conventions and test its ethical standards.

Experts thought the contract for the Joint Enterprise Defense Infrastructure, known by the cinematic acronym JEDI, would go to Amazon Web Services, the dominant player in the field of cloud computing. They did not count on two developments: an extraordinarily aggressive public relations and lobbying campaign by Oracle, one of Amazon’s competitors, and the hostility of Mr. Trump to Amazon and its founder, Jeff Bezos.

On Thursday, the new defense secretary, Mark Esper, said the contract would not be awarded until he had completed an “examination” of the issue. Though Amazon may still win the contract, the decision was seen as a major victory for its rivals: Microsoft, which remains in the competition, and IBM and Oracle, which had been eliminated at an earlier stage by defense reviewers.