Why on earth did we invade Panama?

As we all know, Teddy Roosevelt invaded Panama, partially because of his imperial ambitions and partially because of a possible canal. Then in 1959, we made it a state, along with Alaska, Hawaii, Cuba, and Puerto Rico. But why did we make it a state, and why did Teddy Roosevelt invade the whole country?
 
He didn't invade the "whole" country, he invaded a little part of Columbia. Panama wasn't around until after the war. I guess the point of departure you're looking for is more direct annexation of land. Which would probably entail a real declaration of war not a piddly bit of gunboat diplomacy.
 
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