For U.S. President Donald Trump, Vladimir Putin is a man looking for an off-ramp to his bloody three-year assault on Ukraine.

But according to NATO Secretary General Mark Rutte, the Russian leader may be just getting started. If the alliance does not invest in its defense capabilities, Rutte warned at the annual NATO summit on Tuesday, Russia could attack an alliance country within three years.

By most measures, this year's NATO summit in The Hague was a success. Member states largely agreed to a U.S. demand to boost defense spending to 5% of gross domestic product. Trump, who once derided the alliance as a "rip-off," said his view had changed, while a budding bromance blossomed between him and Rutte, who compared the U.S. president to a stern "daddy" managing his geopolitical underlings.