I would think the eagle represents Germany ( spec. German dominance or German right to dominance)... flying over the west bank of the Rhine (Alsace) establishing that, as the caption indicates, "the Rhine (is) Germany's river, (but) not Germany's border". The "mission" of the German Siegfried-type defender, with the eagle shield, is then pretty obvious.
Hope this helps (..also hope this is at least somewhat accurate )
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