The summit between Joe Biden and Vladimir Putin left two things clear: Russia is almost sure to test Biden’s resolve on cyberattacks, and the U.S. president may find his red line with his Russian counterpart difficult to enforce.

In a meeting in Geneva on Wednesday, Biden said he gave Putin a list of 16 critical sectors that shouldn’t be hacked lest the American government respond with its own cyber forces. But the U.S. has previously struggled to figure out how to curb cyberattacks by U.S. adversaries, and experts said that deterring Russian cyber aggression won’t be easy.

J.J. Thompson, a cybersecurity entrepreneur and adviser, said Putin may attempt to comply with Biden’s pronouncement by "washing” hacking through "affiliates in disparate nation states” to make it harder to attribute the attacks.