What is 'net drain'?
There is an ancient strain in the American political tradition that holds that an active foreign policy is a net drain on domestic priorities, that our allies are merely freeloaders, that we are bankrupting ourselves on military spending, that every U.S. intervention is destined to be a Vietnam-sized disaster, and that the rest of the world should sort itself out.

Thank you.

Question

What is 'net drain'?
There is an ancient strain in the American political tradition that holds that an active foreign policy is a net drain on domestic priorities, that our allies are merely freeloaders, that we are bankrupting ourselves on military spending, that every U.S. intervention is destined to be a Vietnam-sized disaster, and that the rest of the world should sort itself out.

Thank you.

Answer

"net" refers to the gain you make after deducting costs, expenses and losses made in bad deals e.g. net profit.

The figurative meaning of "a drain" means loss, usually in a steady or tiring way.  The physical meaning is to let water run away e.g. through a sieve when you are cooking, or when wastewater goes into the gutter or sewerage system. This gives you a good idea of how the word is used figuratively.

So here, the writer is referring to a belief that, even if there might be some positive effects for USA from foreign intervention, the negative effects are greater.  This means there is a "net drain" for the USA.