This comment.

Well well well, lookit what we got here. A clueless fuckrod decides to try their hand at history. Let's just break this down a bit.

for literally 3000 years

since the time of Alexander the Great

Alexander the Great was born in 356 BC. That's roughly 2372 years ago, not 3000.

Going ahead:

The tribes fight amongst themselves until an outside aggressor comes. Once the aggressor has left, they fight eachother again.

Uhhh...yeah, that's been true of a lot of other nations, not just Afghanistan, although the tone makes it suggest that Afghanistan is especially belligerent in this regard. Take, for instance, France. Soon after the Hundred Years' War, in which the English were the aggressors:

In France, civil wars, deadly epidemics, famines, and bandit free-companies of mercenaries reduced the population drastically.

If you think that I'm going too far back in history, well, that's fair game. After all, I'm actually supposed to consider events from as far back as 3000 years ago.

Continuing with the bad history:

It's hard to compare Syria, or really anywhere in the middle east to Afghanistan.

While that is true, it's not true in the sense that anywhere in the middle east hasn't had internal conflicts after outside military intervention. Consider Iran, after the US organized military coup in 1953:

However the reforms, including the White Revolution, did not greatly improve economic conditions and the liberal pro-Western policies alienated certain Islamic religious and political groups. In early June 1963 several days of massive rioting occurred in support of Ayatollah Ruhollah Khomeini following the cleric's arrest for a speech attacking the shah.

Two years later, premier Hassan Ali Mansur was assassinated and the internal security service, SAVAK, became more violently active. In the 1970s leftist guerilla groups such as Mujaheddin-e-Khalq (MEK), emerged and attacked regime and foreign targets.

Or maybe take into account Iraq, after it achieved independence from the British in 1932.

Establishment of Arab Sunni domination in Iraq was followed by Assyrian, Yazidi and Shi'a unrests, which were all brutally suppressed. In 1936, the first military coup took place in the Kingdom of Iraq, as Bakr Sidqi succeeded in replacing the acting Prime Minister with his associate. Multiple coups followed in a period of political instability, peaking in 1941.

Or why don't we just consider Syria itself?

Syria became independent on 17 April 1946. Syrian politics from independence through the late 1960s were marked by upheaval.

(Note: I do believe colonialism is basically the act of an "outside aggressor," hence I am including that in my examples as well)

I'm only just covering the tip of the iceberg here. There are quite a few examples other than this.

Tl;dr: There are a lot of countries who've suffered from internal conflict after military intervention by foreign aggressors, including ones in the Middle East. Also, Alexander the Great wasn't from 3000 years ago.

Edit: Archived link.

Edit 2: I'd like to thank /u/titich for pointing out the bad history in this post on SRS.