How scared should Europe be of a stoppage in Russian natural gas supplies this winter? That depends on whether countries are willing to sacrifice for one another and unify in their response.

As Russian President Vladimir Putin basked in applause at a Serbian military parade last week, he sent out a clear warning to the European Union: If Ukraine — which has had no natural gas supplies from Russia since June — should siphon gas from transit pipelines to heat its people, he won't make up the difference but will let the EU go short.

When Russia and Ukraine squared off in a similar gas-price dispute in 2009, Putin went further: He cut all gas transit to the EU via Ukraine for about two weeks, forcing Slovakia to declare a state of emergency and Bulgaria to shut down many of its factories. Even Germany had to scramble to make up for lost supplies. So although talks aimed at resolving the energy dispute between Ukraine and Russia resumed on Tuesday, Putin must be taken seriously.