Spoiler: Rules and Laws of Warfare
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Depending on how real-worldy you want to get, you can consider having international treaties in place:


A Ban on RWPs:
The use of reality warping powers (like Wish) are banned by international law. The knowledge and instructions for the Wish spell (and similar) are strictly controlled. A nation would probably have only a few specialists with knowledge of Wish, and they would be watched carefully. Nations would spend large chunks of their military budget on manufacturing Rings of Wish and carefully hiding them in secure locations, like squirrels burying nuclear acorns.


A Ban on SPNs:
The use of self-propagating necromancy(zombie plagues) is a war crime. Research into self-propagating necromancy is illegal except for a few, multinational laboratories which focus on the creation of cures and countermeasures, with strict supervision and careful safeguards. It is generally assumed, however, that most nations have secret laboratories for SPN research.

Occasional, a rogue SPN researcher (evil necromancer) will show himself and nations around the world will send covert forces (PCs) to destroy the rogue researcher and his creations+research.


A Partial Ban on EPCs:
Most nations around the world accept that the use of Extra-Planar Combatants in warfare is cruel and illegal, however a few large nations disagree, either in general, or for specific use cases. The summoning of demons, devils and other extra-planar combatants is thoroughly illegal for civilians, and used only by a few militaries.



Just some ideas on fleshing out international politics and warfare in a theoretical D&Dland.