Trump is a moron, who only believes in bullying -- and is ignorant of the importance of soft power.
Foreign aid like that provided by USAID is primarily a US Jobs program (for farmers), and by providing food to poor nations, the US gained a strategic advantage in the UN. If you have friends, you don't need to bully nations with your military. But Trump is a shithead.
In addition to tanking the global economy, Trump has created a situation where foreign nations won't trust American markets or American investments. Pulling out of the US banking, real estate, and stock markets means a deflation in the value of these systems -- which will increase the length and severity of the economic decline. The fact that no one has given the President lead poisoning is a sign that we're not hurting enough yet -- but we will suffer tremendously.