"We Europeans truly have to take our fate into our own hands," she added.
Special emphasis was needed on warm relations between Berlin and newly-elected French President Emmanuel Macron, she said.
Merkel on Saturday labelled the result of the "six against one" discussion "very difficult, not to say very unsatisfactory".
On a previous leg of his first trip abroad as president, Trump had repeated past criticism of North Atlantic Treaty Organisation (NATO) allies for failing to meet the defensive alliance's military spending commitment of 2.0 per cent of gross domestic product (GDP).
Trump also reportedly described German trade practices as "bad, very bad," in Brussels talks last week, complaining that Europe's largest economy sells too many cars to the US.
Sunday's event saw Merkel renew bonds with the Christian Social Union (CSU), Bavarian sister party to her own centre-right Christian Democratic Union (CDU), ahead of a parliamentary vote in September.
Polls show the chancellor, in power since 2005, on course to be re-elected for a fourth term.