It’s not the fact of the Putin meeting that has European leaders worried—they themselves meet with Putin on a regular basis. Nor is it the timing, which was anticipated given the president’s previous commitments on the continent. The problem is that they cannot anticipate what might come from the meeting. After all, Trump keeps displaying his predilection for keeping even the Europeans off-balance. He slapped massive tariffs on steel and aluminum imports from the European Union and other U.S. allies. He threatened to do the same with European automobiles, risking a full-blown trade war. He departed this month’s Group of Seven summit early, though not before calling for Russia to be readmitted to the group (it was suspended for its invasion and annexation of Ukraine’s Crimean peninsula in 2014); he ultimately retracted his endorsement of the attendees’ joint communique. He has joked that the U.S. should shift its alliance with NATO to a case-by-case cooperation (and reportedly said the military alliance was “as bad as NAFTA,” which is a trade deal).

It’s this kind of unpredictability Europeans fear Trump will bring to Helsinki—perhaps to their peril. “Europeans in general are apprehensive not because they fear an American president dealing with Russia in general, but they fear this particular president dealing with Russia when they don’t know ... his strategic framework,” Daniel Fried, a distinguished fellow at the Atlantic Council and a former U.S. ambassador to Poland, told me. Fried spoke last week to European officials about the summit. “When he seems at odds with his own administration [by] cozying up to Putin and when he seems genuinely attracted to authoritarianism, that is deeply unsettling.”

There are grounds for this concern. In a historic summit with North Korea’s Kim Jong Un earlier this month in Singapore, Trump received from Pyongyang a vague commitment to the goal of denuclearization of the Korean peninsula, without a timeline specifying when this would occur. In exchange, Trump pledged to suspend military exercises with South Korea and gave Kim what was widely regarded as a propaganda victory: The opportunity to be seen not as the brutal dictator of a pariah state, but a legitimate member of the international community.

The risk is perhaps greater with someone like Putin. Unlike Kim, Putin is no international pariah, nor is he new to dealing with U.S. presidents. This president, in particular, has already demonstrated that he’s amenable to pomp and flattery—something the Russian leader has no doubt observed and could try to exploit. “Putin can be charming,” Fried said. “He’s both smart and knows the material. People fear that he could easily get Trump to acknowledge something or give him something for not much, which isn’t good dealmaking—it’s being a sucker.”