1. On September 12, 1846, the United States declared war on Mexico, officially beginning the Mexican-American War. Arizona was still part of Mexico at the time, and the war would lead to its eventual transfer to the United States in the Treaty of Guadalupe Hidalgo.

2. In 1884, the first issue of the Arizona Republican newspaper was published in Phoenix on September 12. The newspaper would eventually become the Arizona Republic, which is still in circulation today.

3. September 12, 1923, marked the opening of the Grand Canyon National Park. The park had been established seven years earlier, but it wasn't until this date that it officially opened to the public.

4. On September 12, 1960, the first-ever televised presidential debate took place between John F. Kennedy and Richard Nixon. The debate was held in Chicago, but it had a significant impact on Arizona voters, as it was the first time that many people in the state had seen the candidates speak live.

5. September 12, 1991, saw the opening of the Desert Botanical Garden in Phoenix. The garden showcases a variety of plants native to the Sonoran Desert and has become a popular attraction among both tourists and locals.