Especially in the North Africa campaign, air units are completely useless. Something about them being at a lower scale seems to make them incapable of ever damaging an enemy, and so I eventually gave up ever using them offensively. The most common result was 3 lost air units to 0 lost targets, and that was when the enemy had no air support. Even beyond that, I lacked the oil to fly air units AND fight with tanks by the time I was at Port Said.
Further, as I previously explained, the weather for Norway means my effectiveness goes down every single turn due to the snow, while the enemy is recovering.
In that turn, when I attacked, I had 2:1 odds.
The next turn when I attacked after using supply trucks, it was 3:2 odds. If I wasn't using supply trucks, it would be 1:1 odds.
Clearly, the one losing more effectiveness is the defender.
On the whole, defense is far, far too strong in this game, and it is only masked by the inexplicably high rate of defenders choosing to retreat or break even when they are clearly winning a battle of attrition. A unit defending, especially along a river, will inflict 8 damage to an enemy while taking 0 damage two or three engagements in a row, then retreat in spite of never taking damage. If you can't get the retreat checks to occur, there is no chance for offense to ever work.