Also, while you two border-sharers are such lovey-dovey friends, he is also a friend of the guy who is sucking up your troops OR friends with the civ that entered the war on your enemy's side. Since you "declared war on my friend," the planning begins. As he builds up, the relative power ratios between you and Mr +15 may approach one another. (Very few of us, even at full war status, can make our power level rise during a good hard slugout at the front. Attacking, especially, eats up units in most circumstances.) While your power ratio may gain relative to the opponent(s), it will drop versus the uninvolved who are also in buildup mode.
And quite rightly. The AI is just not that good at attack, relying on sheer numbers most of the time. 
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