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So it might have been,were it not for a short man, wearing safffron robes and a black felt hat, waving his arms around and shouting:" Terrorism!"
虚拟语气

when the new leader of the free world put all the rancour of the past eight years behind him and heralded an era of unity.
QQ真正的主语是这个吗?

It was meant to be a day of global reconciliation,   main clause
when the new leader of the free world put all the rancour of the past eight years behind him and heralded an era of unity.      subordinate clause

It was meant to be a day of global reconciliation, when the new leader of the free world put all the rancour of the past eight years behind him and heralded an era of unity. So it might have been,were it not for a short man, wearing safffron robes and a black felt hat, waving his arms around and shouting:" Terrorism!"
这应该是一个世界性的“冰释前嫌日”，一个自由国度的新领袖摒除了两国间过去8年间的积怨，宣布一个新联盟的纪元即将冉冉升起的日子。
貌似如果不是一个穿着黄袍子黑帽子，挥舞着手臂大喊：“恐怖主义者”的年轻人，这还能称得上如此完美的一天。

the difficult part should be "So it might have been". looks like sth has been omitted after been.
what logloglog adds when he/she translates the sentence does make sense and  is quite acceptible to me. good job!

It was meant to be a day of global reconciliation, when the new leader of the free world put all the rancour of the past eight years behind him and heralded an era of unity. And so it might have been were it not for a short man, swathed in saffron robes and a black felt hat waving his arms around and shouting: "Terrorism!"
QQ 译文如下:
当自由世界的新领袖(奥老黑)抛却了八年来的敌意,预示着一个和谐世界的新纪元到来之际, 这原本应是一个冰释前嫌的日子. 如果不是因为一个身着黄色长袍,头带黑毛毡帽的小个子男人, 挥动手臂高喊"恐怖主义", 这天本应是一个好日子的.

译完了再看了看原木头的译文,很不错的.