It basically started when Trump repeatedly criticised NATO members on the campaign trail for not paying their fair share and calling it "obsolete". He also mentioned that on Twitter.
Mr. Trump had previously questioned the need for the organization, and on Saturday he reiterated his criticism that other NATO countries were “not paying their fair share” in comparison with the United States.
“That means we are protecting them, giving them military protection and other things, and they’re ripping off the United States. And you know what we do? Nothing,” Mr. Trump said at a subdued rally here on the outskirts of Milwaukee. “Either they have to pay up for past deficiencies or they have to get out.”
“I said here’s the problem with NATO: it’s obsolete,” Mr. Trump said, recounting his comments. “Big statement to make when you don’t know that much about it, but I learn quickly.”
What he mentioned is indeed true as only 5 countries in the alliance meet the target. However, it's also worth noting that the 2% standard is just a guideline, not a legally binding requirement.
Image source: http://money.cnn.com/2016/07/08/news/nato-summit-spending-countries/index.html
Of the 28 countries in the alliance, only five -- the US, Greece, Poland, Estonia and the UK -- meet the target. Many European members -- including big economies like France and Germany -- lag behind. Germany spent 1.19% of its GDP on defense last year and France forked out 1.78%.
Former Presidents George W. Bush and Barack Obama have both also pressed NATO allies to increase military spending, though Trump made that an "essential condition of America’s relationship with the alliance" and consistently pressures members to increase their military spending. He also repeatedly reprimanded members during his foreign trip to Brussels early this year.
Thus, this becomes an important issue in 2017.