Oh my goodness you guys, you're going to put lordlau1 into a depression
On the first turn your invasion force lands in region A. Because you have--more than likely--0 MC (Military Control) in region A when you land there, your invasion force will change to OP (Offensive Posture)--when entering a region where you have <5 MC the moving force automatically changes to OP and gains 5 MC; a minimum of 5 MC will be maintained for as long as you force is in the region--, and it will start gaining additional MC. How much depends on the total police value of your force.
On the second turn your land force marches into region B, while the invasion fleet remains in the coastal region adjacent to A. Because you again enter a region where you have 0 MC your force will change to OP. If there is an enemy stack in region B, your invasion force will attempt to find and attack the enemy stack(s). If you lose the battle and must retreat out of region B the game will calculate into which region your force will be retreated, even if it won't be able to reach it by the end of the turn.
The retreat rules favor certain factors for determining toward which region your retreat will be. One of the greatest factors--if you were moving when the battle occurred--is the region from which you entered the battle region will be
highly favored. Since we are talking about an invasion, and you only have MC in one region nearby, other factors influencing retreat are not of interest to us--eg. friendly depot, friendly city, friendly forces. So if you retreat, it will be back into region A.
If you don't retreat--your force remains in region B--, but you want to break off the invasion, in the next turn you can change your posture to PP (Passive Posture)--or even DP (Defensive Posture)--, and plot a march back into region A by Drag-n-Dropping them into A, and then onto your transports.
If you did retreat after the battle, it can only be into region A, because you
cannot retreat into a region where you have 0 MC, and there is only one region where you have any MC nearby; region A. The turn after the battle, you simply Drag-n-Drop™ your force onto the transports--embarking from the shore takes 5 days IIRC--and sail away into the sunset.
The bad things about invading into a region with an enemy field force, or being caught while embarking, is that your force fights with a great disadvantage.
Never land in a region with an enemy field force, unless your force has a huge size advantage, and you are willing to take some punches, before overwhelming the enemy. If there is only a small garrison inside a city, or even a fort--they are
not in the field--you should have no worries, unless upon seeing your invasion fleet arrive the garrison goes into the field so as to fight your invasion force while it lands on the beaches.