For years, Vladimir Putin's government has engaged in a relentless assault to undermine democracy and the rule of law in Europe and the United States. Mr. Putin's Kremlin employs an asymmetric arsenal that includes military invasions, cyberattacks, disinformation, support for fringe political groups, and the weaponization of energy resources, organized crime, and corruption. The Kremlin has refined the use of these tools over time and these attacks have intensified in scale and complexity across Europe. If the United States fails to work with urgency to address this complex and growing threat, the regime in Moscow will become further emboldened. It will continue to develop and refine its arsenal to use on democracies around the world, including against U.S. elections in 2018 and 2020. The threat posed by Mr. Putin's meddling existed before the current U.S. Administration, and may well extend beyond it. The Russian government's malign influence operations can be deterred. Several countries in Europe took notice of the Kremlin's efforts to interfere in the 2016 U.S. election and realized the danger posed to their democracies. They have taken steps to build resilience against Mr. Putin's aggression and interference, and the range of effective measures implemented by European countries provide valuable lessons for the United States.