As reports increasingly indicate that Russia interfered with the U.S. presidential election to benefit Donald Trump, the president-elect has forcefully pushed back on the intelligence community. Admitting that Moscow played a role in the election, Trump believes, would delegitimize his victory, so he has doubled down on his position that Russia was not involved in the hacks on Democratic Party officials.

Yet for all Trump’s fears about losing credibility, the real threat to his presidency comes not from the evidence presented by the CIA but from his refusal to investigate and respond to Russia’s cyberattacks. If Trump carries out even half of his foreign policy commitments, he will make plenty of enemies abroad by the end of his first term. Those adversaries will seek ways to undermine the president, and Vladimir Putin has just offered a clear example for how to interfere with U.S. politics. That makes it in Trump’s best interest to conduct a thorough investigation into any Russian interference and respond forcefully—before Trump himself becomes the target in 2020.

For most of the Obama Administration, Russia confined its cyber operations against the United States to traditional espionage – using its vaunted capabilities to collect intelligence from the State Department, the Joint Chiefs of Staff, and the White House – all legitimate targets for such activities. As the director for cybersecurity policy at the National Security Council, I saw firsthand Russia’s efforts to infiltrate U.S. systems. Russia’s collection efforts were targeted and often undetected, earning the respect of the intelligence community for their tradecraft and for abiding by a new set of “Moscow Rules” that governed the new great game of spying in the digital age.