Which was the last state to join the union?

Hawaii was the last state annexed to the United States, officially on August 21st, 1959. Although it already belonged to the United States since 1898, but not as a state.

It should be noted that before these events, this archipelago was constituted by a sovereign Polynesian kingdom with a relatively stable monarchy despite its relationship with other non-Polynesian cultures, until it was overthrown by the europeans in 1893, after which it was declared as the Independent Republic of Hawaii until 1898.  

It was there when the United States intervened, because by that time there were a considerable number of American families descended from the missionaries and farmers who worked in Hawaiian soil in the cultivation of pineapple and sugarcane. Therefore the US government took control of this archipelago as territory, but not as a state, which meant that the people of Hawaii did not enjoy the autonomy to elect their own rulers and were not taken into account in the decisions.

It was not until August 21st, 1959 that Hawaii was declared a state, becoming an area of great economic, tourist and military importance and the farthest incorporated territory of the United States. In addition, it is the only state that is in Oceania and is entirely composed of islands.

c) legal battles last five weeks after the election due to the narrow victory in several states.