Your use of the singular verb "is" suggests that Trump has some unified, coherent foreign policy. Perhaps you think he is a noninterventionist. He is not. He is a wholly transactional person who makes decisions based on whatever will personally help him. If he thinks invading will help him, he will. If he thinks staying out will help him, he will. It is not as though there is some bedrock principle guiding his decision-making. Simply put, he has no foreign policy. There will never be a Trump Doctrine.