@SgtPepper:
But still I am a little bit confused about scrambling. Why can’t I and my allies send planes to defend army from enemy’s tactical bombers if it is quite logical and realistic.
Planes in a territory or sea zone are part of the combat in that area, so they do defend land units in the same area against attack.
However, if you are questioning why they cannot defend a neighbouring territory in the same why that they can defend a neighbouring sea zone, then I guess the simple if unhelpful answer is 'cos that’s the rules! Someone else may be able to provide a reason why it makes sense. Others may tell you that they have varied this rule for their own games, as you could. Being a learner with 1940 I am sticking to the rules myself.
@SgtPepper:
Oh yes, and I’ve got a new question… can I send bombers to attack only an enemy airbase with planes except land units?(I mean…just want to destroy enemy air fleet rapidly…but sure opponent will put all damage on infantry)
The defender chooses which units to remove in combat. So if you are attacking air units in the same area as land or sea units your attack will need to be strong enough to kill the cannon fodder before it can get to the high value units.
Attacking an airbase is different - see Strategic & Tactical Bombing Raids in the rule book. But this does not involve attacking air units, unless they choose to intercept, as per Barney’s answer below.
Enjoy the game SgtPepper! I certainly do!