Do a math. European military build-up with that size of spending, far more than that of US, is strong enough for Europe to protect themselves without much help from US. 5% spending makes Europe grow up to become independent.
Trump's another goal is NATO countries have to buy military equipments from U.S.. Most of them don't have capacity to make these equipments anyway. So, the end results of 5% spending: Europe take care of themselves and increase US arm exports. Trump is genius.
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