I suspect that one of the signs of the end of any civilization is the growth of expediency and the loss of principle.
Principle, as you might suspect, is making decisions based on some objective truth: this thing is right, or this thing is wrong. It is a logical framework by which one can make choices which reflect a coherent world view and allows one to move towards larger goals.
Expediency, on the other hand, is making decisions purely based on one's own perceived self interest at a point in time. It cares not about the whether something is right or wrong all the time; it only cares what is right or wrong in relationship to one's position and one's objective. There is no logic applied - except the logic of what one perceives to the best outcome for one's self. It could be construed as a form of "The ends justify the means."
So, for example, principle states that offering potential aid and comfort to a country's enemies is always wrong (principle). Expediency states that it is fine to attack and berate the president I do not like for potential collusion with the enemy, but no to attack the president I do like for stating (when he thought he not on-mike) "I'l have more flexibility after the next election."
The split can run deep - for example, berating one side when a candidate pushes a reporter but quietly standing by when the side I support also pushes a reporter, or decrying the potential for civil unrest while actively keeping quiet when civil unrest perpetuated by my side breaks out.
Why does this such an insidious development? Because expediency will, in the end, ultimately trump principle in the short term for the simple reason that if one side practices expediency, the other side will ultimately be forced to adopt expediency (decisions that benefit them) to survive - or quietly be wiped out of existence. On the other side, a series of expedient decisions seldom creates a pattern of sustainable growth and survival: one bounces reactively from one point to another until one is trapped in a box of one's own making (after all, if every one is making expedient decisions you are no more than one obstacle to some one else).
Sadly, in the attempt to gain power and prestige through expediency, those that use it find that they are only tearing themselves apart with the own hands, leaving nothing but a wasteland for someone else to build on.