I'll second Draugnar's answer.

To further clarify, it is the TERRITORY that is attacked, not the army or fleet. You move from Burgundy to Paris not because you're spoiling for a fight with the army in Paris, but because you wish to capture the territory of Paris itself. Therefore if, in the same move, the army that started out in Paris wants to move to Gascony, there is no need for it to wait around in Paris to 'defend itself'.

And yet, should it fail to move to Gascony, it still has no problem defending Paris as if it were just holding. A little bit non-intuitive, but very important to the mechanics of the game.

Well, let's say they get chased out of Gascony, and dash back to Paris just in time to man the barricades!

From behind?

it woudln't be much of an attack if the army coming up from behind caused it to cancel.

that would mean you couldn't move armies forward one after another...

now that means an army can attack one territory while holding another, but only if it doesn't take it, so your army moves to invade, while other elements hold back, while invading you force that other army to stay home and defend (cutting any support, unless it is one of your own units)

it is a bit of an abstraction but this simplification is really important for the game mechanics to work.

Interesting - I always found it was actually very intuitive.
The way I see it is when you say 'ATTACK!!!' your men go and line up on the border. There they have a fight with the next bunch along. If they win this they then progress over the border and leave their location empty.
If they do not win the fight there are still in their original territory, and are perfectly capable of turning round to defend themselves - after all its much easier to defend than attack.
Breaking support is even more sensible: Men line up at border to help their comrades, then get attacked from behind and have to turn and defend.