You may be thinking that the attack on A could help defend A if additional units were supporting the move:
Country1: Unit A SUPPORT <something>.
Country1: Unit B -> A.
Country1: Unit C SUPPORT B -> A.
This is true, but C could have just as easily supported A to hold, which would have been just as effective. Any unit which can support B -> A can also support A to hold, so unit B's order does not add any value.
What other purpose could the order to unit B serve?
