When the constitution was written, The section regarding titles of nobility also mentions what government employees can and cannot do. Emoluments are one of the things that are forbidden. What are they? These can easily be defined as benefits from foreign governments that allows said employee to personally profit from.
As the owner of a business, President Trump is to find ways to maximize his profit and make transactions to do just that. He hasn't been extremely successful at that nor at many other things in his life. When he chose, upon taking office, to not put his interest in a blind trust, then his ownership of the Trump Organization would be subject to constitutional law.
Since taking office, his company has benefitted from deals funded by the governments of the UAE, Saudi Arabia, and China. The Indonesia development recently got a 500 million dollar boost from the Chinese government. Shortly after, Trump pledged to help with the ZTE production and talks of a trade war have suddenly cooled.
In the previous administrations, Qatar was our regional ally in the war on terror. Hosting the U.S. military personnel, it provided a gateway to launch missions into Iraq and Syria. Currently because of the lack of diplomatic relations between the Saudi, Emirati and Qatari governments. Trump pick the side that would profit him the most. Of course, that turned into early deals for Trump and Trump-related projects.
This man can be easily bought and it slowly becoming an issue regarding national security. Not only does love the ignorance of so many, he profits from it.