The president said he had a "very good relationship" with embattled British Prime Minister Theresa May.
Trump should not further insult and alienate the Europeans.
"If tomorrow there is an increase in tariffs, like in the car industry, our reaction should be united and strong to show that Europe is a united and sovereign power."
Trump boasted that the NATO leadership is very fond of him, but says Russia must be thinking "if we did like this guy, we made a big mistake."
It was the first time Russia attempted to use violence outside of the former Soviet Union to achieve its political aims, a report said.
The letters said America's contribution to the NATO alliance is "no longer sustainable."
The 68th Tank Regiment was renamed for Berlin and an army regiment was renamed for Warsaw.
"We want to send a clear message to any potential adversaries that NATO is there to protect all allies against any threat," said NATO Secretary General Jens Stoltenberg.
This is the first time Belarus has joined Russia and Serbia for their annual military drills, which will feature some of Moscow's new military equipment.
Russia has added over a dozen new submarines to its ranks since annexing Crimea.
Bureaucracy, aging infrastructure and incompatible equipment are plaguing the U.S. and NATO's ability to reach parts of Europe eyed by Russia.
President Donald Trump's national security adviser sharply criticized the Russian president in the past.
Russian submarines are more active in Europe than at any time since the end of the Cold War, Admiral James Foggo said.
The U.S.-led NATO Western military alliance and Russia accused each other of fueling the Cold War–like crisis across Europe.
The news comes during a period of heightened tensions between Russia and NATO. Moscow's neighbors have expressed concern about Russia in the wake of the 2014 annexation of Ukraine's Crimean peninsula, as well as Moscow's support for pro-Russian separatists fighting in Ukraine.
Oslo announced this week that it would ask the U.S. to substantially increase the number of Marines it stations in Norway.
Russia has consistently denied any efforts to disseminate "fake news" to impact foreign nations.
Tensions are rising between NATO and Russia.
There will be plenty for world leaders to tell Donald Trump at the upcoming G7 summit in Canada.
Secretary James Mattis is pushing for NATO to establish 30 land battalions, 30 air fighter squadrons, and 30 navy ships, including destroyers, to be ready to launch a fight against Russia.
America is leading a drill with 18 other nations in Europe's northeast.