"If you look at NATO, where Germany pays 1 percent and we are paying 4.2 percent of a much bigger GDP – that's not fair," Trump said.

Last year, Germany spent 1.13 percent of GDP, far below the NATO target agreed upon between member states in 2014 in Wales. In the agreement, member states were given until 2024 to reach that goal. Berlin has vowed to follow through on that commitment.

How Germany achieves this target is a contentious subject domestically, with arguments that often fall along party lines.

German Chancellor Angela Merkel and Defense Minister Ursula von der Leyen of the Christian Democratic Union (CDU) have backed a straightforward interpretation of the NATO target and promised to meet that goal by 2024.

Outgoing Foreign Minister Sigmar Gabriel of the Social Democrats (SPD) has argued that the target should be revamped to include other factors, such as humanitarian aid and stabilization programs, which he says also contribute to defense and security.