What was the main reason imperial nations built large naval forces?
to initiate sea-based attacks on other nations
to demonstrate their size and prowess
to protect their economic interests
to resist attacks on their harbors

The main reasons that imperial nations built large naval forces was to protect their economic interests in these colonies as well as their political interests. In addition they also sought to demonstrate their size and prowess in their ability to exert military force around the globe. 

The main reasons that imperial nations built large naval forces was to protect their economic interests in these colonies as well as their political interests. In addition they also sought to demonstrate their size and prowess in their ability to exert military force around the globe. 

Bis your answer and your welcome