The Prince discusses principalities (ruled by a single man), not republics (democracies), and provides policy recommendation. It is addressed to Lorenzo de Medici, a duke from a ruling family, and closes by advising him to organize Italian troops against hostile barbarians.
Hereditary rulers benefit from stability and can more easily retain control of their state and enjoy public favor.
Colonies are "economical, reliable, and do not give excessive grounds for resistance." Subjects of monarchies are used to obedience, but subjects of republics will never forget their former liberty and will try to oust their occupier. Territories submit more easily when they share the same language and customs as their ruling state and are geographically close to it. The colonizing king must eliminate previous rulers and shouldn't impose new taxes or laws on the territory if he wishes to have their favor. It is best if the ruler goes to live in the new territory, where he can supervise more effectively and win his subjects' love and fear. No one will criticize the ruler's attempt to win new territory unless the necessary military capacity is lacking.
The private citizen can rise to power in four ways: by fortune, virtue, nefarious action, or popular support. Of those who ruled by virtue or skill, the best were "Moses, Cyrus, Romulus, Theseus, and others like them."
If a ruler subdues his subjects with violence, the violence should decrease, not increase, over time. Public opinion responds well to harm done quickly and good done slowly.
A ruler should aim to set policies that need never be changed. His foundation is his subjects and he should seek their favor rather than that of the power-seeking elite. He should take care that citizens do not avoid business or property improvements for fear of taxes.
Ecclesiastical states should not be debated because they were built up by God.
A ruler should give all his attention to matters of war. Mercenary soldiers are to be avoided, as their commanders seek their own power and the soldiers will desert. Auxiliary armies are a burden. A ruler should know when to fight lawlessly like an animal.
A ruler should have a reputation as a great man. Some vices are essential to a ruler's "welfare and peace of mind." For example, it is better for a ruler to be thought miserly than generous, and it is not bad to be perceived as cruel if the perception results in keeping the public in order.
Rulers should be suspicious of their subjects and of other rulers.
It is always better to be an ally or an enemy than to be neutral. The neutral party faces the wrath of both winner and loser.
Every decision requires risk assessment.
A ruler cannot rely on the advice of others; he must be wise enough to know when to take advice.
Niccolo Machiavelli. The Prince. (1516) Translated by David Wootton. Cambridge: Hackett Publishing Co., Inc. 1994.
This summary was written in 2005, along with a series of other 500-word summaries of philosophy books, as an exercise in brevity.