Ms Merkel had already begun finessing her message on Monday, stressing that she was a "convinced trans-Atlanticist", a message she repeated after a meeting with visiting Indian Prime Minister Narendra Modi in Berlin.
But growing frustration with U.S. President Donald Trump, a determination to reform Europe with France's incoming leader and political considerations closer to home convinced Merkel to take a stand, senior German and European officials said.
Merkel, addressing a election campaign at a beer tent in Germany's southern state of Bavaria, said on Sunday that following the election of Trump and Brexit, Europeans "really have to take destiny into their own hands".
Trump also has demanded that Germany meet the North Atlantic Treaty Organisation military alliance's defence spending target of two per cent of gross domestic product and criticised Merkel's open-door migration policy. Germany and France, two strong advocates of the landmark Paris climate change accords, oppose the Trump administration's initiative to potentially pull out of the agreement.
"We are meant for each other", Mr Modi said.
Trump believes that European powers owe much to the United States, particularly in the areas of defense and trade. "The times in which we could rely fully on others-they are somewhat over", Merkel said. "The West has become smaller, at least it has become weaker", Gabriel said.
Merkel said she reached that conclusion after the G7 and NATO Summits, in which Trump took part and during which the differences between the policies of the new United States administration and its allies in Europe and elsewhere became patently obvious.
German Chancellor Angela Merkel says Europeans must take their fate into their own hands.
"He has a lot of respect for her", Spicer said.
The discussions on Europe's defense have become more active following Trump's victory in the USA presidential election in November.
In Washington, Trump swiftly reacted to Merkel's comments.
"Those who have accompanied Chancellor Merkel journalistically for a long time know how important the German-American relations are".
"He said they're very bad on trade but he doesn't have a problem with Germany", said Cohn, director of the National Economic Council, according to the White House pool report from Taormina, Sicily where the G-7 summit was held.
Following the G7 summit, which concluded at the weekend, Mr Trump has said he needs more time to decide if the USA would continue to back the Paris climate deal.