The allies cannot control Mr. Trump’s messages or Twitter outbursts, but they can control their own, analysts say. Preparing for Mr. Trump is mostly about strategic messaging in the room — what he is told — and ensuring the right strategic messaging outside the room, the official said, especially with Mr. Trump scheduled to meet the Russian president, Vladimir V. Putin, in Helsinki next week.

A core element of deterrence is making it clear that the political will exists to use military force if NATO is challenged — a more difficult sell when the United States president appears to be more in conflict with his NATO allies than with Mr. Putin, the leader of the nation NATO is intended to deter.

As one indication of how to handle the criticism, the Canadian prime minister, Justin Trudeau, who Mr. Trump derided as “dishonest and weak” at the disastrous Group of 7 meeting last month, and who received one of the more aggressive pre-summit letters from Mr. Trump about increasing military spending, is visiting Latvia before coming to Brussels.

Although Mr. Trump’s letter did not mention it, Canada took the leading role in Latvia in one of NATO’s new “spearhead” commands, which are based in the three Baltic nations and Poland and aimed at Russia. Ottawa has also committed to increasing its defense budget by more than 70 percent over the next decade.

Germany, too, promises to increase military spending to 1.5 percent of its economy by 2024. While not the 2 percent level, Berlin will argue that will still be more than any other NATO country other than the United States. Mr. Trump, who appears to have a special animus toward Germany, believes that Berlin has developed a vibrant social system and thriving export-driven economy unfairly and on the back of the United States, by not spending enough on defense.

Germany is currently spending 1.24 percent of gross domestic product on its military, which will rise to 1.31 percent next year, an increase of $5 billion to $43 billion, a sizable if still insufficient increase, said Norbert Röttgen, chairman of the German Parliament’s Foreign Relations Committee.