The early days of Númenor are full of blissful peace, great comfort and abundant wealth. However, around 2,000 years later, things start to get chaotic. The people, especially kings, begin to desire the immortality that is denied to them. This splits them into two factions: the King’s Men, who cling to life and plot against their elven allies, and the Faithful, who cling to old alliances.
Naturally, with the kings and their supporters leaning to the discontented side, the geopolitical behavior of the nation begins to change. The Númenóreans begin to become more imperialist, building settlements in Middle-earth, harvesting natural resources and forcing local Men to pay homage to them. Nonetheless, Númenor continues to thrive – like many. So much so, in fact, that their wealth and power quietly paved the way for their demise.
See, despite their success, the Númenóreans leave one thing out of their calculations: Sauron. The problem isn’t that they don’t know the Dark Lord. On the contrary, in the middle of the Second Age, Sauron forges the Rings of Power (including the One Ring) and attacks the Elves to claim the other Rings. In this conflict, the Dark Lord has almost won, but the Númenóreans arrive just in time and crush his armies. This does two things: it gives Sauron a special grudge against the Men of Númenor, and it gives the Númenóreans a superiority complex.
Throughout the rest of the age, Sauron begins to fight with the Western Men, attacking the Númenorean settlements on the coast. Finally, a king named Ar-Pharazôn the Golden takes over, and he hears that Sauron calls himself the “King of Men”. Taking great umbrage at the thought, he embarks with one goal in mind: to subdue the Dark Lord himself.