I have a good idea about how we can solve the problem of what to do when an enemy takes your resources from you.  It also provides some very interesting information for the info cards and also simplifies unit creation.  If you've ever played The Sims 3, you know what the Moodlet system is.  It's an icon that applies some set of bonuses/minuses to your player character (or in this case, your unit).  In Elemental, you almost certainly want to call moodlets something else, but for this discussion, I'll keep using the name.

The moodlet system would work very well in Elemental.  In fact, I would propose that ALL of a unit's stats should be derived from its moodlets.  For example, you might give your unit plate mail, which would give it a "heavily armored" moodlet.  The icon for "heavily armored" (maybe a big breastplate) would show up on the card, and mouse over of the icon would show that this adds 10 to armor (or whatever).  You might also give it training, giving it the veteran moodlet.  Thus, a chevron goes on the info card and mouseover of that shows that 10 hitpoints are provided. 

You could also have a moodlet "level up" tree similar to Civ4's unit leveling.  Perhaps some of your units are better at defending cities than normal.  Or defending the forest, etc.

So, how does this help with enemy resources?  Well, if you don't have a resource, then your unit could just get another moodlet when it's created: "Under equiped" which reduces hitpoints by some percentage.

Moodlets could also be added or removed by the use of spells.  A spell might grant a flying moodlet.  Or an extra hitpoints moodlet.  And a quick mouseover of the card would show where these extra hitpoints are coming from.

So the design would be as follows:

Equipment/Training/Current Conditions yield Moodlets, which in turn yield the unit stats.