It would be difficult to defend the U.S. action in acquiring the Canal as ethical. U.S. interest in the canal was the result of a book by Admiral Alfred Thayer Mahan, The Influence of Sea Power on History. Admiral Mahan argued that if the United States was to become/remain a great power, it would need, among other things, an isthmian canal. At the time the canal was under consideration, Panama was part of Colombia. The terms of the Bidlack Treaty stated that the United States would guarantee Colombian sovereignty over Panama; and the Clayton-Bulwer Treaty between the U.S. and Britain stated that no canal would be build across Panama without mutual consent. This was later modified to remove that annoying obstacle.
The United States offered Colombia $10,000,000 plus an annual lease payment for the right to finish the canal, which had been begun (and abandoned) by the same French company that had dug the Suez Canal. Panama refused, and asked for $25,000,000, in response to which President Theodore Roosevelt flew into a rage and made several racial remarks about the Colombians.
The Panamanians staged a revolt against Colombia, and the United States conveniently placed warships near the border to prevent Colombian troops from reaching the area by sea. Since they could not cross the jungle to get there, the revolt was quickly successful. A representative of the French Company, one Bunau-Varilla, who had travelled to Washington and apparently arranged for the U.S. warships to be deployed returned to Washington as the new ambassador from Panama. On his arrival, President Roosevelt recognized Panama's independence. In exchange for the recognition, Panama granted the U.S. the right to dig the canal.
Colombia was deeply offended, and broke diplomatic relations with the U.S. Later, during the Warren Harding administration, the U.S. paid Colombia $25,000,000 because the U.S. needed Colombian oil; but did not apologize.
An ironic note: when President Carter agreed to return the Canal to Panama, a Republican who opposed the move commented, "we stole it fair and square; now Carter wants to give it back."