President Trump has arrived in Hamburg, Germany, to attend this year's G-20 summit.

At meetings Thursday and Friday, Trump will join other world leaders to discuss issues of international consequence.

Here are some things to watch for.

First off, the protesters. As I noted a week ago, tens of thousands of protesters have made their way to Hamburg to protest the summit. Many of these protesters are peaceful, but some are not. The potential for an embarrassing incident — such as protesters obstructing a motorcade — is higher than it would normally be. Expect Trump's Secret Service detail to stay especially close to the president at all times. Also watch to see if Erdogan's security detail go nuts, as they did in Washington not long ago. If they do, Turkey may have its future G-20 invitations suspended.

Next up, Angela Merkel. The German Chancellor is hosting the summit and as such, has set the agenda. But she has two options about how to proceed with that agenda. She will either attempt to build consensus around a G-20 statement of principles ("we agree to pursue X etc."), or she will try to isolate Trump. Merkel is furious with Trump for his criticism of her immigration policies and his rejection of the Paris climate accord. Yet where some European leaders, such as President Emmanuel Macron of France, have attempted to extend a warmer hand to Trump, Merkel has thus far done the opposite. Merkel is unlikely to criticize Trump by name, but if she implies a negative connotation towards the president, he may respond. That would send the summit careening off course. Russian President Vladimir Putin would love that. And speaking of Putin...

Vladimir Putin and Trump. I will write more about this later on Thursday, but watch for the body language between the two leaders. Putin will almost certainly be effusive towards Trump. He wants the U.S. president to believe that he's a good guy who just has honest disagreements with the west. In that dynamic, Putin hopes he can manipulate Trump.

Xi Jinping and the China identity. President Xi of China is the new hero of the western liberal elite. It's the Paris climate accord, stupid. After all, with Xi committing China to reducing its emissions in the long term, European leaders are fawning over him. It's pretty pathetic that an authoritarian communist is the belle of the ball. It also says a lot about European diplomacy that they trust Chinese leaders to keep their word ( they won't).

Theresa May and Trump. The British Prime Minister is under pressure at home and abroad. In London, she faces a Parliament that doubts her leadership (the doubters include many Conservatives). Abroad, she faces increasingly challenging negotiations with the European Union. As May attempts to get the best possible Brexit deal, her E.U. counterparts want to deny her any solid victories. In short, she needs a win. And Trump might be the man to deliver it. The key factor is whether May and Trump release a statement offering a firm timeline for a U.K.-U.S. free trade deal. May is desperate for that deal because it would help consolidate British business interests. And Trump has shown sympathy for getting a deal done sooner rather than later.

Of course, G-20 summits are often unpredictable. But because of the unique array of personalities and diverging policies that will meet in Hamburg, this summit is sure to be more interesting than most.