Narated By Anas : Allah's Apostle said, "Help your brother, whether he is an oppressor or he is an oppressed one. People asked, "O Allah's Apostle! It is all right to help him if he is oppressed, but how should we help him if he is an oppressor?" The Prophet said, "By preventing him from oppressing others." - Sahih Bukhari Volume 003, Book 043, Hadith Number 624.

"While non-retaliation against a personal injury is frequently a virtue (see the Holy Qur’an, chapter 41 verse 34), Islam believes that human communities have the right to collective self-defense, since non-resistance to aggression would result in a world dominated by tyrants (see Holy Qur’an, 22:40). Under some circumstances, Muslim scholars will allow oppressed peoples to rebel against their oppressors. They might, therefore, classify the American War of Independence as a form of jihad, broadly understood. When Bosnia was faced with ethnic cleansing in 1992, the Muslim authorities there authorized the use of force to defend the country’s Muslim minority. The alternative would have been mass murder and mass rape, and therefore jihad was lawful. Furthermore, some Muslim scholars will permit a non-defensive ‘idealist’ war to establish justice and freedom in a neighboring country. This is analogous, perhaps, to the decision of the United Kingdom to declare war on Germany on September 3, 1939, in response to the German invasion of Poland. There are more recent analogies as well, including very recent instances in which Western powers have used force to overthrow tyrants such as Saddam Hussein."

(citation: http://ageofjahiliyah.wordpress.com/2007/09/01/abdal-hakim-murad-on-jihad-apostasy-rights-of-muslim-women/)