Axis & Allies Developer and Playtester; War of the Ring Editor and Playtester
From the FAQ:
With fighter strafing, can I only try and move in eight units, or can I keep moving in units until I get up to eight units?
You must declare all the units that are entering a particular zone before moving any of them. Your declared move must be legal under the stacking limitation; you can't declare that ten units will move into a zone with the expectation that two will be destroyed by enemy fighters. If the zone already contains four units, you can't designate more than four more units to move in. If two of those units are destroyed by strafing, then you're left with six units in that zone.
In the case of tanks, you only need to declare their movement when resolving movement into their final destination. For example, assume there's a German tank at Villers-Bocage that the German player wants in Ouistreham. It must move through Caen to reach its destination. The German player doesn't announce that tank's move along with other units moving into and stopping in Caen. He declares the tank's move when he's declaring other moves into Ouistreham. Any fighters in Caen get to strafe the tank as it enters and leaves Caen.