Russia and Ukraine: How do we know the war has begun?

Everyone is trying to speculate about Putin's intentions towards Ukraine. The United States withdrew family members of its embassy staff as fears escalated of a conflict erupting there. But that conflict may already have begun, writes security and defense expert Jonathan Marcus.
The dangers of an all-out war between Russia and Ukraine grab the headlines.

And all the obvious questions are asked: Will Russia go on the offensive? Is President Vladimir Putin insisting on the war, whatever the results? Or can diplomacy bring peace?
But we cannot know what is going on in President Putin's mind. And there is another question: How will we know that the war has begun?
The tanks are moving, the missiles are going off

The answer seems obvious. Russian tanks massed on the border between the two countries cross the border of Ukraine, or a barrage of missiles or air strikes targets Ukrainian positions. Certainly, this will be a dramatic escalation of the crisis and we will enter a new phase of the conflict.