Determined to stop Russia's interference in the presidential campaign, at least one of President Obama's senior advisers urged him to make the ultimate threat to Russian President Vladimir Putin, U.S. officials told NBC News: Mess with the vote and we will consider it an act of war.
In October, the U.S. used the latest incarnation of an old Cold War communications system — the so-called "Red Phone" that connects Moscow to Washington — to reinforce Obama's warning that the U.S. would consider any interference on Election Day a grave matter.
Part of the message sent over the Red Phone on Oct. 31, according to a senior U.S. official, said: "International law, including the law for armed conflict, applies to actions in cyberspace. We will hold Russia to those standards."
The so-called "Red Phone" system is used to communicate in moments of crisis. Formally known as the Nuclear Risk Reduction Center line, it is no longer a literal phone, and instead sends email messages and attachments.