Think of it as the new equivalent of the East German-West German border. It was across that border that NATO (i.e. US) and Warsaw Pact (i.e. Soviet) forces glared at each other, constantly ready to leap into what probably would have been global annihilation.
We've moved the line. It's now the border of Russia.
NATO is set to agree on a major boost to its defences in eastern Europe including six bases and a "spearhead" force of 5000 troops in response to what it called Russian aggression in Ukraine.
Secretary General Jens Stoltenberg insisted that the moves likely to be approved by defence ministers meeting in Brussels were purely defensive, but the increased presence on Russia's borders is likely to rile Moscow.
With Eastern European states concerned by the worst tensions with Russia since the Cold War, the bases will be in the alliance's three Baltic states -- Estonia, Latvia and Lithuania, where the alliance has had little presence until now -- plus Poland, Romania and Bulgaria.
The new NATO-Russia front line won't have an invasion-capable force deployed, at least not yet. Just a few thousand troops for starters. Yet it's the sort of thing that's quite susceptible to 'mission creep.' If Putin responds in kind with a NATO-targeted force, we'll be tempted to up the ante and from there it becomes a game of escalation. Before long you have theatre-nuclear weapons and tank armies pointing at each other across an invisible line and a lot of people holding their breath, hoping that no one does something stupid.