General antonyms:
* Nationalism: Emphasizing unity and shared identity within a nation.
* Unity: A state of being joined together as one.
* Harmony: A state of peaceful coexistence and agreement.
* Cooperation: Working together for a common goal.
* Integration: Bringing different groups together.
More specific antonyms:
* Federalism: A system of government where power is shared between a central authority and regional units, often promoting collaboration.
* Universalism: A belief that all people are equal and should be treated equally.
* Internationalism: Cooperation and collaboration between nations.
The best antonym for "sectionalism" will depend on the specific context in which you are using it.