The wasteland began way before the fall of civilization.
It started with the widespread use of Ignium in the 2050s, and its continued use through the 2060s and 2070s. Ignium, an energy similar to electricity and plasma, is easy to create and extremely malleable, therefore economically better than electricity. Its downfall was its slight toxicity. With its widespread use by billions of people, Ignium slowly poisoned the soil, air, and worldwide ecosystem, leading to cataclysmic climate change and leading to the sixth mass extinction on earth. Combined with pollution such as trash and other waste, the oceans were killed, the sun was blotted out, and the soil itself became a mix of dirt, Ignium, and plastics.
Ignium’s usage became a dependency, and by the time that it was discovered to be extremely damaging to the planet it was too late. Many major companies depended on it and funded campaigns to cover up the damage it caused. During the 2070s, the world became destabilized and eventually collapsed. The population soared to roughly 14 billion, countries collapsed from resource, water, and food shortages. Despite the amazing technology of the 2070s, major parts of the population died due to illness and starvation as unemployment (in the US) soared into 60%. Toward 2078, Europe collapsed into war between its nations while the US fell into a three-way civil war. At the same time as a world-wide pandemic and world-wide economic collapse, this broke the country. The pandemic which started in India, ultimately left India, the Middle East, and Africa in a broken state with most of the population dead or dying.
During 2079, the last powerful countries on Earth (such as the USA, Russia, and China) declared war on each other over the last resources on Earth. After a few nukes, bombs, chemical weapons, mutagenic bombs, and more, civilization finally came to a crumbling halt.