11/9 of all days?

I believe they were making a passing reference to "9/11".

So why is the author using 11/9?

because some countries, like England and much of Europe, write the date before the month. Others, like the US write the month first. You just have to get used to it. If the author talks about "September 11" instead of "11 Septermber" you would kind of expect him or her to write "9/11", but he/she does not seem to be big on cinsistency.

I see. But previously he mentioned November 9, the date when Berlin Wall fell down. I just don't understand why he mentioned "9/11"(September 11).

The issue being discussed is 'events which have defined the course of international relations'. The author is pointing out the coincidence of the the date Sept 11th. for two such events, both of which concerned the demolition of a symbolic structure; the Berlin Wall - Communism, the Twin Towers - Capitalism.

But the Berlin Wall fell on November 9, not September 11.

The author is just pointing out that they are inverted for no apparent reason. It's just confusing writing.

Many thanks!

It's a little late to point this out, but the numbers are not inverted because of the difference between European and American styles of writing the date. Both dates are given in the American style, which is the day of the month first, then the number of the month. The Berlin Wall came down on November 9, (11/9), and it is a historical event of much importance.
The World Trade Center attack was September 11, (9/11), and it also is a historical even of much importance. And that phrase "9/11" became adopted as the label for that event.

It's just a remarkable coincidence that two very important historical dates, well within the memory of most people alive today, happened to occur with a date of such similarity.