inJESUS, the Caliphs do not represent Islam and should not be looked towards to understand the religion. Firstly because they were not appointed by the prophet to represent the religion. They merely appointed themselves to the throne and as you put it “waged oppressive wars” not to spread Islam but to spread their power and influence. These are the same people who conspired to Kill the prophet on more than one occasion because of their hunger for power and greed.
Many at the time felt that leadership of muslims should have remained within the immediate family of the prophet and this caused great troubles and in-fighting among the muslims.
For this reason the Omayad and Abbasid dynasties sought to misinterpret the quran and fabricate hadiths in the favor of the Chaliphs and companions by hiring paid fabricators and scholars, which allowed them to create an alternative source of Islamic authority to the holy family of the prophet. They branded this false authority with titles such as “the rightly guided Caliphs” or “the righteous companions”. Authority not sanctioned by Allah and the prophet.
keep in mind these oppressive wars were not waged on non-Muslims alone. They were waged on muslims who opposed the Caliphs and refused to recognize their authority. Entire tribes were massacred and many more imprisoned.