I believe the clash of civilizations is a reality. I think there is plenty of proof in today's society. For example, when people from different civilizations/ancestries come together, humanity has always had a problem with seeing the opposing group as equal. This is seen with Roma people in Europe, blacks and Mexicans in the United States. Imagine what it would be like if you weren't fully aware of the other civilization.
When two civilizations clash, it is usually over a fear of the unknown. These fears are part of humanity and have been with us for thousands of years. This fear leads to hate which leads to war, violence and suffering. The clash of civilizations is inevitable as long as humans continue to give into their basic fears of the unknown. When those fears subside, then we will all be united in peace.
Rome clashed with the Gauls, and other traditional clashes are inevitable. But what can be "controlled" or improved upon is diplomacy. It is likely that lower classes may continue to fight and such, but diplomacy stops civilizations from two things that should be considered = ethnic cleansing, and nuclear war.
Its completely true that people from different cultures clash - something that is essentially just describing a clash of civilizations on a smaller scale. People meet each other with different norms and ideas of how to act, so its only natural that conflict ensues to some degree. Still, this clash doesn't have to be permanent in the least, and people should focus instead on how co-understanding.
There is currently a major clash of civilizations occurring the world. As more Muslims immigrate to Europe, there are many disputes about what is considered acceptable in society. Many Muslims want to adhere to traditional cultural mores, while native Europeans have a much more progressive stance towards their lifestyles and habits.