MANCHESTER, N.H. — The 2016 presidential race will be remembered for many ugly moments, but the most lasting historical marker may be one that neither voters nor American intelligence agencies saw coming: It is the first time that a foreign power has unleashed cyberweapons to disrupt, or perhaps influence, a United States election.

And there is a foreboding sense that, in elections to come, there is no turning back.

The steady drumbeat of allegations of Russian troublemaking — leaks from stolen emails and probes of election-system defenses — has continued through the campaign’s last days. These intrusions, current and former administration officials agree, will embolden other American adversaries, which have been given a vivid demonstration that, when used with some subtlety, their growing digital arsenals can be particularly damaging in the frenzy of a democratic election.

“Most of the biggest stories of this election cycle have had a cybercomponent to them — or the use of information warfare techniques that the Russians, in particular, honed over decades,” said David Rothkopf, the chief executive and editor of Foreign Policy, who has written two histories of the National Security Council. “From stolen emails, to WikiLeaks, to the hacking of the N.S.A.’s tools, and even the debate about how much of this the Russians are responsible for, it’s dominated in a way that we haven’t seen in any prior election.”

The magnitude of this shift has gone largely unrecognized in the cacophony of a campaign dominated by charges of groping and pay-for-play access. Yet the lessons have ranged from the intensely personal to the geostrategic.