Genghis Khan has a reputation as a ruthless warlord and genocidal maniac, but I've heard rumors of him being a politician interested in unifying local tribes and bringing peace to the region. Was there a noble motivation to his campaign?
I think it is important to consider an arc of development for Gengis Khan and not to consider his actions in isolation.
First would be the early childhood and youth phase where Genghis Khan, who was only known as Temujin struggled to survive in a tough political climate of Mongolia, surrounded by constant warfare, raids, blood revenge, and dire poverty. To this must be added constant interference from outside interests, such as China, who would manipulate one tribe against another, causing constant instability.
Next is the period during which he united the Mongol tribes. This was done in two ways. His closest friends and allies were promoted in a form of meritocracy and rewarded for their loyalty. Others were recruited as allies and yet others were tricked or manipulated. The end result, however, was an unprecedented unification of Mongolia that hugely benefitted the Mongolian people. You have to realize that at that time, with tribal thinking dominating all forms of organization in Mongolia, this was a huge and difficult feat.
Next came the period of conquest and this needs itself to be divided into two phases.
In the first phase, Genghis Khan invaded China, and though not conquering the whole of China, laid the foundations for the future Yuan dynasty that his successors created. Perhaps the results here though positive from a Mongolian perspective were mixed from a Chinese perspective, with a loss of pride and control of China by the Chinese.
The second phase consisted of the invasion of Persia and all the lands to the west of Persia. This period started with Genghis khan trying to deal with the Shah of Persia, but when his ambassadors were rebuffed (or killed), Genghis Khan descended on the continent, obliterated cities, took men and boys as soldiers and pressed the remaining population into slavery. The histories of the Persians therefore view Genghis Khan as a bloodthirsty tyrant, and in fact the term Genghis is derived from the persian word Changiz, which means the claw of the tiger, referring to how a tiger will rip apart its prey. However, you have to note that from Changiz Khan's perspective, brutality was an effective strategy as many cities simply capitulated at the first sight of his army.
All in all though, Genghis Khan, like many emperors before or after him started off faced with the central problem of how to create a united land that can guarantee the security of his people; like so many other emperors, the expedient answer was war and invasion, which perhaps for the time was the only method. And like so many emperors, once he started invading and had success, the question of security morphed into something more, perhaps seeking glory and a grand place in history, especially given that the people of the region were well aware of Alexander the Great's conquests and measured greatness against what Alexander had conquered.
First of all, never had Genghis Khan been some kind of maniac. He was a very reasonable man, you can figure this out by reading The Secret Chronicle of The Mongols.
The reason to conquest Jin dynasty in Chinese is to secure the newly formed unification of Mongol people because Jin dynasty had long been pressuring Mongol tribes. The reason to conquest Khwarezm Empire in Persia is because they had shown disrespectful manners to Mongol envoys. Genghis Khan kindly requested the local official who was responsible for massacre of Mongol envoys. However, they refused and behaved further disrespectful. Also, reasons to enter Central Asian Kara Khitan and Middle Eastern Baghdad were based on the request by the local people because they wanted protection against runaway Naimans and Hashashin (Assassin) respectively.
Genghis Khan's "noble cause" was his fight for the "underdog." That is, he waged war on large, proud empires such as Jurchen (Manchurian) China, Persia, and others, on behalf of (initially) weak and poor Mongolian (and other) tribes. In doing so, he brought down some major tyrannies (although often, "the cure was worst than the disease").
After the smoke of battle had cleared, he brought religious and cultural tolerance to the whole Mongolian empire, and trade and educational opportunities to poor (even to this day) people of central Asia. This was, however, at the expense of richer nations like Persia, Russia, and China.
Genghis Khan created a system of advancement based on merit, rather than birth. This was most apparent in the appointment of his army commanders at all levels. He also took the best artisans, scholars etc. of his conquered countries to his capital at Karakorum to work for his "government."
Basically, Genghis Khan ruled better than he conquered; the reverse was true for many others.
It would be quite difficult to determine his exact reasons for his actions, so we have to look at his specific actions, and the reasons behind others' similar actions. Note: this is all speculation.
We know for a fact that he conquered large swaths of land, the nobility of which is a matter of opinion. However, with this example, you must keep in mind the fact that many emperors with similar actions were done for gain of personal power, there were few emperors who conquered massive areas of land only for their nations, and for no personal gain.
However, we also know he was relatively tolerant to people of other religions and cultures. This may indicate that he really did want to unite the tribes around him.
So he may have done it for personal power, and he may have truly done it for nationalism.