As President Trump prepares to meet President Vladimir V. Putin of Russia in the coming weeks to discuss a possible cease-fire, a senior German general warned that if Russia is not stopped in Ukraine democracy in Europe would be at risk. “The only reason for Putin to stop is if he is stopped,” said Lt. Gen. Alexander Sollfrank, and that requires pressure. He had a similar job at NATO and earlier fought in Afghanistan as commander of a rapid-reaction force that saw serious action. He spoke in an interview last week at his Berlin headquarters. As Mr. Trump tries again to wrangle Mr. Putin into a cease-fire in Ukraine that Russia does not want without significant Ukrainian concessions of territory, General Sollfrank said that the accomplishments of the postwar world were at risk if Russia prevailed in Ukraine.