In his tweets, he also attacked Obama, questioned the FBI, torched CNN and bragged about his 2016 win.
Trump and the first lady also had tea with the queen in their first official visit to Britain.
Trip planners have taken care to keep the president from the demonstrations.
May had rolled out the red carpet and praised the special relationship.
Trump said U.S. support for the alliance continues — claiming countries offered up more money.
But he added that if the Russian president denies involvement, “What am I going to do?”
The soccer manager has inspired the nation.
The U.S. president began his week of diplomacy with a harsh rebuke.
Some of those who devised the pledge say it was never intended to be used this way.
Hard-line backers of Britain’s split with the European Union claim that the British government is reneging on promises for a clean break from the bloc.
They fear he could target them and ally with Putin.
Dawn Sturgess, 44, was one of five people who became seriously ill after being exposed to Novichok in the Salisbury area.
Both Germany and France are turning immigrants away.
A giant ‘Trump Baby’ balloon has been permitted to fly above London.
Investigators follow two worrisome scenarios: remnants from a high-profile March attack on a former spy or a previously undetected remnant of the Soviet-era Novichok agent.