First claim: exaggerated
In the three decades before Trump's election, NATO spending declined by two-thirds.
On page 4 of this 1987 report(pdf) on NATO military spending, we learn that in 1987 they were globally spending 5.0% of their GDP on defense. More precisely:
- USA: 6.6%
- Canada: 2.1%
- (Western) Europe: 3.4%
In 2016, i.e. year 1 before Trump, this level was down to 2.49% of GDP.
The military spending has thus decreased by a half, and not by two-thirds, in share of GDP.
In nominal terms, of course, it had not decreased at all, since GDP more than doubled from 1987 to 2016.
Please note that these comparisons are not on a consistent number of countries, since NATO expanded to new countries after the end of the Cold War. I suspect than Warsaw Pact countries, e.g. Poland, were spending a huge share of their GDP on defense, but it would be complicated, and certainly meaningless, to refine the figures to include them.
Second claim: true
[Before Trump came to office] only 3 other NATO countries were meeting their financial obligations.
At the 2014 summit in Wales, all of the NATO members agreed to spend 2 percent of their GDP on the defense by the year 2025.
NATO published the report Defence Expenditure of NATO Countries (2013-2019)(pdf)
According to table 3, Back in 2016, only 3 NATO countries beside the USA were fulfilling this 2% commitment: Greece, UK, Estonia were at least at 2.07%, and Poland stood just under the threshold with 1.99%.
So, 3 out of 27 non-US NATO countries, the claim is correct.
Third claim: true
The number of NATO allies fulfilling their obligations more than doubled (all-caps changed to lowercase).
In 2019, spending is just an estimation, but beside the US, 8 countries pass the 2.0% threshold:
- Country / Total Spending as Share of GDP
- Poland 2.00%
- Latvia 2.01%
- Lithuania 2.03%
- Romania 2.04%
- United Kingdom 2.13%
- Estonia 2.14%
- Greece 2.28%
- Bulgaria 3.25%
Bulgaria announced a spectacular increase in military spending, it was standing at 1.48% in 2018. This is due to a huge one-time command in 2019 (cf notes below), so it is hard to imagine how much they will spend on defense in 2020.
If we include Bulgaria and the USA, the number of countries above 2.0% of military spending passed from 4 to 9, so this is indeed more than doubling.
However, this is maybe not as significant as the capital letters might make the readers of Trump's tweeter account think: for instance, Poland's military spending has increased from 1.99% to 2.00% of GDP. The figures are more impressive for Romania's (from 1.40% to 2.04%) Latvia's (from 1.45% to 2.01%), and of course Bulgaria.
Fourth claim: rather false
NATO spending increased by $130B
Defence Expenditure of NATO Countries (2013-2019)(pdf) estimates that NATO total Defence expenditure (table 2), in constant dollar, raises from $913B in 2016 to $987B in 2019. This is a $74B increase, slightly more than half of what Trump claims.
Using current prices and exchange rates, however, the increase is +$128B, from $911B in 2016 to $1039B in 2019. This might be the figure Trump refers to, but it is misleading. The extra $54B are due to inflation and to the exchange rate of US dollar($) versus other NATO countries currencies, mainly Euro(€).
In any case, about half of the raise is due to the US own growing military budget. Canada and Europe NATO countries have raised their military spending by $40B in this 3 years period. (Which I would describe as significant, but also significantly lower than the figure given by Trump's tweet).
If we were Since we are on Politics.SE and not Skeptics.SE, we might discuss how much of this raise is due to Trump's diplomatic effort, and how much was already planned (the 2% commitment was taken in 2014, countries needed time to converge there), or a reaction by East-European countries to the perceived threat of Russia after the Ukraine crisis, or a consequence of NATO members putting less trust in their American ally for their own protection.
Montenegro joined NATO in 2017, mechanically raising the total military budget of the alliance. This amount of circa $70M, however, doesn't affect the general picture.
Bulgaria announced a doubling of its military expenditure in 2019, which would make it pass the 2% threshold. @T.E.D gave the explanation in a comment: Bulgaria's blip up this year is due to a one-time purchase of 8 expensive F-16's (at about 15mil a pop).
Writing this answer feels weird, because this is the first time I fact-checked a Trump tweet that happens to deserve the 'rather true' mention.