Half of the US’s 3.5% of GDP defense spending must relate to its other spheres of influence, such as the Pacific and the Middle East, including standing bases that can’t just be moved elsewhere at a moment’s notice. So if other NATO countries started spending 4% of GDP on their armed forces, Europe would quickly exceed the US in military capability in the Atlantic.
To pick up on one of the countries already exceeding the 2% target, UK GDP in 2016 was US$2.619 trillion, so raising its defence spending from 2.1% to 4% of GDP would cost almost an extra US$50 billion, or £37.5 billion at current exchange rates. That’s two Brexit buses’ worth a week. Let’s fund our NHS instead.