Extremists during WWZ
A militant from the White Supremacist and Neo-Nazi organization, the National Alliance, aiming down the sights of his AR15 at a Zombie in Tennessee, circa 2014. During WWZ, White Supremacist and Neo-Nazi groups like the KKK, National Alliance, and many others would view the crisis of WWZ* as an opportunity to secede from the Union to from their own Ethnostates and would come into conflict with Government Troops, bands of survivors, bandits, and with the Zek Hordes. They would also engage in various horrid atrocities against non-whites, Jewish people, captured Government troops, and other people they didn't agree with. During the Road to New York Campaign, the US Armed Forces would be engaged in a brutal conflict with these groups, though they fought a long hard conflict, the US Military would ultimately defeat these groups by 2024.
* = These groups would view the Solanum Virus and either part of "A plot by the International Jew" or a "Deadly consequence of Multiculturalism and Altruism."
Serbian Ultranationalists battling with NATO forces near Foca, Bosnia, circa 2022. As the Great Panic plunged the world into chaos, one of the badly areas was the Balkans where the chaos and the subsequent Zombie Conflict reignited tensions in the region which had been around since the last war there in 2001. During WWZ, large scale warfare between the various ethnic factions would occur there as well them fighting the local Zek hordes. Since the early 2020s, NATO had been engaged in activities there in an attempt bring law and order back to the region, though they would have limited success in doing so as of 2028.
An armed band of Marxist Revolutionaries in Colombia, date unknown. WWZ would bring severe instability in Latin America, thus making a perfect recipe for Neo-Marxist Revolutionaries in the region to rise up and attempt to take control of the local governments. These groups would view the Zombie War as the "Great Collapse of the Capitalist World Order" and the people of the world would soon follow to bring about "A Worldwide Communist Revolution." As the late 2020s, these conflicts between the governments and these Communist Revolutionary Groups is still ongoing. These groups would also come into conflict with hordes of Zeks within Central and South America.
(Credits to IMFDB on their page about Invasion USA (1983).
(Note: I do not condone or support any of the groups mentioned in this post or their ideologies.)