Josh Marshall at Talking Points Memo says it best

Some people think Trump has no actual foreign policy. This is not true. He is extremely ignorant. But he has an instinctive and longstanding way of thinking about and approaching foreign policy questions which goes back decades before he ran for President. It is one that sees international relations in zero-sum terms (for me to win, you have to lose), sees the US as being taken advantage of by allies (either through advantageous trade deals or expenditures on defense). This is why you see economic nationalism going back decades with Trump and either skepticism or hostility toward international treaty organizations like NATO.

Now, in practice this can mean opposing the Iraq War, supporting the Iraq War, depending on how things are going at the moment and the state of public opinion. But this prism through which he sees the world (not unlike the way he approaches business, political campaigns, etc.) is consistent over time. What you also have in Trump is someone who is impulsive and aggressive by nature - you see these qualities in primary colors in everything he does. These are highly dangerous qualities in a President. They become magnified when such a person is being advised by people who provide an ideological purpose and justification to such impulsiveness and aggression.

That is where I fear and believe we are with Trump. Not everything in foreign policy is sacred. But here we have an impulsive and ignorant man whose comfort zone is aggression surrounded by advisors with dangerous ideas. His instinctive aggression makes many of their most dangerous ideas possible; and their ideological formulations give his actions a rationale and logic that transcends psychological impulses and the anger of the moment. Even President Bush had a coterie of more Realist-minded and cautious advisors to balance out the hotheads. They lost most of the key debates - especially in the first term. But they provided a restraining counter-balance in numerous debates.

At present there is no one like that around Trump at all.