ALTHOUGH PRESIDENT TRUMP likes to rely on his instincts, this week’s meeting with Russian President Vladimir Putin in Hamburg, Germany, calls for careful preparation and straight talk. Mr. Trump’s national security adviser, H.R. McMaster, told reporters that “we have no specific agenda” and “it’s whatever the president wants to talk about.” This is far too casual and risky.

While Mr. Putin’s actions at home and abroad are often objectionable, an exchange in person with him can help avoid mistrust and misperceptions, of which there are plenty. Mr. Trump should set aside his stated admiration for Mr. Putin’s strongman tendencies and instead confront the Russian president with difficult questions. This meeting is not about being friends but about urgent business. The agenda is rather full.

Mr. Trump simply cannot fail to admonish Mr. Putin for Russia’s attempts to meddle in the 2016 presidential election. He must make clear the United States will not tolerate it, period. Naturally, this is a difficult issue for Mr. Trump, who reaped the benefit of Russia’s intervention and now faces a special counsel’s investigation, but nonetheless, in his first session with Mr. Putin, the president must not hesitate to be blunt. He should not be overeager to give back the two Russian compounds used for espionage that were seized by the United States in December in President Barack Obama’s belated response to the election meddling.

On Ukraine, Mr. Trump must also display determination. Russia fomented an armed uprising and seized Crimea in violation of international norms, and it continues to instigate violence in the Donbas. Mr. Trump ought to make it unmistakably clear to Mr. Putin that the United States will not retreat from the sanctions imposed over Ukraine until the conditions of peace agreements are met.

The leaders ought to discuss the Syrian conflict with an eye toward avoiding direct hostilities, even as Washington and Moscow pursue dramatically different military goals. Mr. Trump should at least try to persuade Mr. Putin to acknowledge the need for a government not headed by Bashar al-Assad and a region not dominated by Iran. Mr. Trump might also fruitfully bring up an idea floated recently by former Democratic senator Sam Nunn and former Russian foreign minister Igor Ivanov, among others, to restart broader Russian-American military-to-military communication. It would also be in the interest of both countries to resume cooperation on nuclear nonproliferation and to resolve the standoff over the Intermediate-Range Nuclear Forces Treaty.

Even in the darkest days of the Cold War, the United States and the Soviet Union communicated with each other, and the need is no less today. A meeting will probably satisfy Mr. Putin’s desire to be seen as a global leader, and he will be probing Mr. Trump for signs of weakness. Mr. Putin suffers from long-standing misunderstandings about the West and the United States, and it can only help to speak to him directly, if the message is carefully prepared.