She told allies they should work together to "raise the cost" of malign behaviour whenever it occurred, calling out cyber-attacks, taking action against intelligence networks of hostile states and bringing in tough sanctions against aggressors.
A "family photo" from the North Atlantic Treaty Organisation summit in Brussels perfectly captures the awkwardness and tensions overshadowing this year's gathering.
But Trump's increasingly vitriolic tone on those issues, plus his criticism of Merkel's migration policies and his decisions to pull out of agreements dear to Germany like the deal meant to prevent Iran from getting a nuclear weapon and the Paris climate accord, have many questioning what comes next.
While NATO members increased overall defense spending in 2017, for a second consecutive year, the United States was still largely outspending all other members.
On "America's Newsroom", former USA ambassador to the United Nations and former New Mexico Gov. Bill Richardson (D) said meeting with Putin is the right move for Trump. The chancellor, who grew up in the former East Germany, told reporters, "I have experienced myself how a part of Germany was controlled by the Soviet Union".
The president pointed to Germany as an example and said the country was "totally controlled" by and "captive to Russia" over the Nord Stream 2 pipeline project, which would "double the amount of gas Russian Federation can send directly to Germany, skirting transit countries such as Ukraine", FoxNews.com reports.
"When Germany makes a massive oil and gas deal with Russia", Trump said to Stoltenberg.
"This has gone on for many presidents".
Republican Sen. Orrin Hatch also took issue with Trump, saying "I don't agree with that".
Trump's decision to meet Putin was widely regarded as a blow to the prime minister's attempts to isolate Russian Federation following the Salisbury attack in March, even though he had strongly condemned the Kremlin's actions at the time.
"These countries have to step it up - not over a 10 year period, they have to step it up immediately", Trump said. German Chancellor Angela Merkel has said that the project is merely commercial, but the US and other European Union members believe the pipeline could be a geopolitical incursion by Russian Federation. "After the attacks of 9/11, we called on Germany's leadership to fight with us in Afghanistan. He diminishes alliances we built to safeguard an economic and strategic force that has allowed millions of people to live in freedom", Kerry continued.
Though the insult will hardly engender warmth from German chancellor Angela Merkel, who Trump is due to meet later today, German defense minister Ursula von der Leyen seemed relatively unperturbed.