Mattis, for example, was kept in the dark as Trump made the decision to withdraw the US from the Iran nuclear deal in May, NBC News reported on Monday. The Pentagon chief learned of the decision from a colleague and tried to get the president on the phone before a formal announcement was made.
Trump also did not discuss with Mattis his decision to halt US military exercises with South Korea, which was announced following the landmark summit in Singapore with North Korean leader Kim Jong-un earlier this month, the network said, Presstv Reported.
Last week, Trump again blindsided his defense secretary by publicly directing the Pentagon to establish a sixth military branch overseeing operations in space.
"They don't really see eye to eye," said a former senior White House official.
The way these recent policy decisions have played out underscores a significant change in Mattis's role in the administration, with Trump relying less on the advice of one of the longest-serving members of his cabinet, officials said.
It also represents a significant departure from Trump's early enthusiasm for the retired four-star Marine general he proudly called "Mad Dog."
While Trump and Mattis have had disagreements over some issues from the beginning, the president still kept his defense secretary in the loop on major policy decisions.
In recent months, however, Trump has soured on Mattis, in part because he believes the Pentagon chief looks down on him and slow-walks his directives, officials said.
The dynamic was exacerbated with Trump's decision to appoint hawkish John Bolton as national security adviser, which Mattis opposed, as well as Mike Pompeo's confirmation as secretary of state.
Trump is now more inclined to rely on his own instincts or the advice of Pompeo and Bolton on major policy issues, sources told NBC.
The development comes as Mattis is embarking on a tour of Asia amid increased tensions with China over trade and what officials call militarization of the South China Sea. He will be visiting Beijing this week.
Mattis’s mission comes as the Trump administration is set to start taxing $34 billion in Chinese goods, while Beijing has promised to impose retaliatory tariffs on American products.