Irredentism refers to the acquisition of land included in another country by reason of cultural, ethnic, historical, and other ties. Although few, if any countries are currently explicitly mandating irredentism, the complication of having ethnic communities divided by borders created by colonialists is hugely troublesome. As is the more obvious hardship of having to create a nation-state out of many different nations due to different cultural groups being jumbled together into one state.
Speaking of potential explosive dangers in the Western African region of the Sene-Gambia, Centre for Democracy and Development writes, "Under French Colonialism, the major economic interest was in the groundnut producing northern region [of Senegal] and the Casamace area [in the South, below The Gambia which bisects Senegal] was neglected. In addition, the Jola people of the Casamance were not part of the Savannah and Muslim culture of the rest of the country so cultural difference was compounded by political neglect even after independence.
"President Jammeh [of the Gambia] is from the same ethnic stock as the rebels of Casamance and it is suspected that he has also been involved in funding the insurgents in that troubles part of the country."
It should also be noted that the separatist movement in the Casamance is the longest running civil conflict within western Africa. Although this is not irredentism, it's its antonym -- caused by the same factor of inappropriately created borders.
Implications from Institute for the Future:
The issue of culture is being more widely recognized as important when dealing with conflict resolution. This can be evidenced by the increase demand for anthropologists in peacebuilding mechanisms. Even if we were to account for the economic asymmetries in conflict settings--which for the case of Sene-Gambia includes north/south difference mentioned earlier, President Jammeh of the Gambia funding the Casamance, the arms shipments to Casamance rebels by way of Guinea Bissau-- that tend to be the focal point of conflict discussions, a case can be made that culture and identity issues loom large in the Sene-Gambia conflict.
The same can be said for many conflicts within Africa, for example much of the conflict in the Horn of Africa due to the creation of Somalia as a state containing only about two thirds of ethnic Somalis. The rest live within Ethiopia, Kenya, and Djibouti. Much of the fighting between Somaila and Ethiopia, until as recently as 2009, was due to Somalia's desire to reclaim Somalis living in Ethiopia.
Center for Democracy and Development, Nov. 2010 pg. 8: