I too have discovered that setting all or most units to "DEFEND ON LINE" is a successful way of inflicting many casualties on the enemy, without many of your own army routing, (except the inevitable cavalry units that go charging off all over the place).
But the problem I'm finding with this is that after all of the nearer enemy units have been decimated and have routed I wish to move my "DEFENCE LINES" nearer to the remaining enemy.
But when left clicking on any corps commander the only order options that remain are "TACTICAL BOMBING", "HALT PROGRESSION", and "RALLY ALL UNITS".
The original orders of "DEPLOY ON LINE", "DIVERSION ON LINE", DEFEND ON LINE", and "MARCH TO AREA" have all disappeared, and don't come back again. So the only thing I have been able to do, in these circumstanses, is to End Game.
Am I missing something, or have I done something wrong. Any help would be much appreciated.