Now being a few weeks into my History of Western Civilization: Ancient to Medieval, I have started questioning the reasoning behind expanding empires. What prompts an empire to conquer thousands of miles of land? Were kings trying to become famous? Did they believe they could rule the world? Were they trying to promote the empire's beliefs? Did expansion promote popular opinion of the current ruling entity?
Where I struggle with this question is that empires seem to fail when they grow to such large sizes. With the speed limitations of ancient times (news, troop movement, etc), it would get harder and harder to control your borders and people the larger the empire grow. So it would seem that if you were to expand, you were to call upon your own demise.
Since ancient history is a decent chunk of time, I have been studying the Egyptian, Persian, Greek and Roman empires so answers in relation to those four would be appreciated. I realize that the Egyptians and Greeks did not focus on expansion but using them as a comparison in your answer will resonate with me better than, say, medieval France.
There are many reasons kingdoms would conquer vast tracts of land for themselves. Land used to equal power. The more land one held, the more power they held. In addition to the power factor, there is the argument of natural resources. Take Great Britain for example during the Imperialism Era. They had land all over they globe for natural resources. They controlled the East Coast of the North American continent for the timber resources to build and maintain a massive navy. Not to mention the competitive factor. They had been going at it with France for centuries, and once France left North America, forcefully, Britain controlled most of the continent, again relating to power. The New Imperialism was more about natural resources than a global pissing contest, but the power factor was still present. The more land ones holds, the more power one holds. There are a host of other factors, but I simplified it so I didn't write a novel.
I have written on (historians' view of) the motivation for the expansion of Rome here. It's partly fear (sometimes irrational fear) of neighbours, but mostly greed and glory which led to Roman expansion.
Note I have summarised decades worth of debate in that last sentence; read the link for a bit more detailed and nuanced overview of the debate - the link also includes sources to the most important works on this subject.