Why did european nations form alliances in the early 1900s?

The main reason why European nations formed alliances in the early 1900s was that "They were unsure they could protect themselves if attacked by larger nations," since nationalism had caused nations to develop an "us vs. them mentality". 

to act quickly in times of danger to the united states

because you have to act quickly if you want to live/survive.

the correct answer option is: weapons.

explanation:

tensions rose as the two opposing superpowers competed for 'weapons'.

referring to the cold war (1945-1991), the two superpowers, usa and ussr, experienced a phase of struggle and conflict with each other. these two superpowers considered each other as a threat to their survival and thus opted for policies to strengthen their positions.

the two superpowers never went to war with each other directly but competed for weapons to preserve their positions.