USD is accepted all over the world. U.S can easily print USD to buy up the most valuable assets from all over the world. Ethics aside. What is preventing the U.S from doing so? What did I miss out?
To a certain extent it can. But not too much, else it will create a run on the dollar. If speculators no longer trusted the dollar, they would sell it for other stores of value (like Euros, gold, etc...). Foreign dollars would have to come home and be redeemed by real wealth which means less real wealth for the US domestic economy and inflation. This can feed on itself and cause a self-reinforcing inflationary fire that would be difficult to put out.
Any increase in the money supply causes an increase in prices (inflation), which is exactly the same as saying a reduction in the purchasing power of the currency. If foreigners believe the USD to have lower purchasing power, then they'll want more USDs in exchange for a unit of their currency (or in exchange for their stocks, bonds, physical assets, goods, services, prized assets etc).
If the federal reserve printed a LOT of new USD, then the effect would be very noticeable.
In case you're wondering why the USD hasn't depreciated a lot since 2008 (when the fed began very loose monetary policy including huge amounts of quantitative easing, i.e. printing more money), that's because other countries (especially US trade partners) were affected by the global recession too; so their central banks engaged in similar policies causing the USD to appreciate back up to pre-2008 levels.