Tillerson will reportedly miss the meeting so he can be in Florida when President Donald Trump hosts Chinese President Xi Jinping.
Germany spent $37 billion on defense in 2016 and has pledged to increase that amount over the next few years.
President Trump and German Chancellor Angela Merkel don't always see eye to eye, and sometimes, it's gotten personal.
Officials have said it won't be a "European army," but others argue it sounds a lot like NATO.
Citing Russian aggression in Ukraine and Crimea, Sweden has decided to bring back its military draft.
During his appearance with NATO's secretary general, the vice president also commented on the former national security adviser's firing.
German Chancellor Angela Merkel said her country will do more to meet NATO expectations for defense spending.
James Mattis, a retired Marine Corps general, has historically been pro-NATO.
South Carolina Gov. Nikki Haley is up for the position of U.S. ambassador to the United Nations.
Some people waved and held up American flags as U.S. troops in tanks and other vehicles crossed into southwestern Poland from Germany.
One of the largest units at Fort Carson is deploying to Europe for nine months.
Obama touted the strength of the United States' alliances with European nations during a press conference with the German chancellor.