Complex sentence: Roosevelt felt that this increase in military and demonstrating its power in a war would greatly increase the Unites State's precense on the world stage and give the country a foreign policy which it had been lacking.
Split into simpler sentences: Roosevelt felt that this increasing military strength and demonstrating its power would greatly increase the Unites State's precense on the world stage. Also Roosevelt was looking to give the country a foreign policy, something which it had been lacking.