But despite the doubts, the US "will always be there for NATO, even if they are not there for us," he continues.
The president further writes that the United States has gotten NATO countries to agree to increase the target for individual countries' defense spending from 2% to 5% of GDP, which was decided this summer.
"Everyone said it was impossible, but it wasn't, because above all, they are all my friends," Trump writes.
The US relationship with NATO has once again come under the spotlight following Trump's statements that the US "needs" Greenland, citing national security.
