The sin of sinas chinum, (translated as “baseless hatred,” “needless hatred,” and “unwarranted hatred”) was given by our sages as the root cause for the destruction of the second temple.  It is frequently invoked as an admonishment for how we treat our fellow Jew.
But common attempts to explain this term as strictly referring to Jewish divisiveness and lashon hora (evil speech) specifically during the second commonwealth do not add up historically.
During the first commonwealth, there was idol worship and murder, a split in the kingdom, and a civil war.  Do we really believe there was less divisiveness and evil speech than in the second commonwealth? 
Even during Moshe’s reign, there were detractors and even a full scale attempt at rebellion.   
What was unique during the second commonwealth was not signature Jewish divisiveness, but rather, striking Jewish unity and unchecked power during the peak of the fundamentalist Hasmonean regime. And it was their sinas chinum, and the sinas chinum of zealous Jews in subsequent generations, that led to a continued deteriorating situation with Rome, and the subsequent destruction of the second commonwealth, in varying and increasingly severe stages.
The Hasmoneans sure did hate the Greeks.  And there were certainly good reasons to hold a grudge.  But the Greeks were defeated, even if only to the point the Americans were defeated in our conflict with Cuba.  Even today, the U.S. military base remains.  But unlike Castro, who continues to accept Guantanamo Bay as a reality he must swallow, the Hasmoneans were blinded by their hatred, and invited Rome in to drive out the Greeks, even though the Greeks were no longer the threat they once were, nor did they have the designs for us they once did. 
And Roman intervention was (surprise, surprise) not for free.  The Greek base was merely replaced with a Roman one.  And we were now on the expansive Roman map. 
Even the pro-Roman King Herod can be traced to Hasmonean sinas chinum, as he was a paternal descendent of Hasmonean conversions by sword.   So too, after eventually being conquered by the Romans, many Jews continued to resist out of bitter rage, consistently rejecting realpolitik.  It had all the success of pulling on a slip knot.  The destruction only increased in severity with each ill-fated rebellion. 
As we look with anger towards Lebanon, Syria, and Iran, it is important that we do not emotionally focus on the wrongs that are being done to us, or even the wrongs that are intended for us. 
It is simply not a luxury that a small civilization with one tiny country can afford.  We always have to think rationally, and not set policy out of anger. 
It is not enough to condemn sinas chinum towards each other.  We must not have sinas chinum towards others.  Even towards our enemies.