Human beings need enemies -- sometimes for good, other times for ill. But having an easily definable enemy is often very helpful.
Organizations need enemies to send you mail and call your home asking for donations. The March of Dimes was established by Franklin Delano Roosevelt to combat the crippling disease of polio. With the advent of the Salk and later the Sabin vaccines, polio was effectively wiped out in the United States and the March of Dimes needed a new cause. It found one in preventing birth defects and later expanded into helping women have healthy pregnancies.
During World War I and World War II the enemies were easy to identify. They wore uniforms that clearly showed their allegiances, and combatants generally stayed on their side of the battlefield.
Sometimes friends can become enemies. Hitler and Stalin signed a pact to carve up Eastern Europe and leave each other alone, but Hitler decided that his empire just didn't look tidy without the Soviet Union, so he wheeled his troops around to the east and tried to defeat Stalin.
On the other hand, sometimes enemies become friends and allies. After starting two world wars in thirty years, Germany decided it was better to outwork its competitors rather than outfight them, and now is among the most influential nations on the planet.
I raise this because, as we head into uncharted waters in Syria and with a newly elected president in Iran, enemies are harder to identify and friends are harder to retain.
A great deal of the weaponry we provided to the Mujahedeen in Afghanistan (friend) when they were fighting the Soviet Union (enemy) fell into the hands of the Taliban (enemy) and have been used to some effect against NATO forces (friends) over the past decade or so.
There is some concern that, if we provide weapons to the Syrian rebels (friends) and they win, they may be overtaken by a branch of al-Qaeda (BIG enemies) and we will have, in effect, provided weapons to our most virulent enemy.
Egypt used to be a good, good friend even though we were a little uneasy about the way President Hosni Mubarak spent the billions of foreign aid dollars we sent him.
Now that Mubarak is out, Egypt (sort of friend) is being run by a government largely made up of members of the Muslim Brotherhood (neither enemy nor friend).
While all that is going on, Turkish Prime Minister Tayyip Erdogan (friend) has his hands full with young Turks rioting Erdogan's continuing efforts to turn Turkey from a secular nation (good) to an Islamist nation (not-so-good).