Because 3% would already be enough to make the US irrelevant for European security. At that level of spending, the combined military power of European NATO memb

Because 3% would already be enough to make the US irrelevant for European security. At that level of spending, the combined military power of European NATO member states would eventually be comparable to the US. If Trump kept insisting for more, European leaders would just call his bluff. And maybe even secretly hope that the unreliable partner would leave NATO.