Why don't more countries spend 2% of GDP?

"Twenty-three of the 28 member nations are still not paying what they should,"

Trump told heads of NATO states assembled Thursday in Brussels.

Many expertspoint out that the 2%  target is problematic.

NATO has warned against a rush to spend for the sake of spending

emphasizing that budget decisions must be based on strategic planning..

http://money.cnn.com/2017/05/25/news/nat...ned-trump/

NATO is based on the principle of collective defense:

an attack against one or more members is considered an attack against all.

So far that has only been invoked once

-- in response to the September 11 attacks !

Trump is upsetting all of America allies