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I played this one very often, so i might be able to help.

In this game, there is no opportunity to play the allies in campaign mode. This was made possible in the sequel, Allied General. But apart from having to play the Germans Panzer General is by far better then Allied General. In AG, missions are shorter and smaller which makes them less fun and more difficult. In fact I almost always managed to conquer Washington in PG, but in AG I never got any further than Italy. Besides AG is not as playable(?) as PG.
And for that random thing: It is most probable that you were caught by rugged defence. The huge readme.txt of the original game said that this means that your enemy lets you come quiet close and then opens fire suprisingly. This can only be done by entrenched units, most probably infantry or antitank units in cities or forrests. To avoid this, you can either dig them out by artillery or bombardement from the air or by attacking with engineers for they never suffer from "rugged defence".

Hope i could help you.

P.s.: Btw is PG 3 abandonware yet?