I commented at length in response to this when it was still listed on the Question Answer Board. To continue that thought, I agree with the previous respondent that U.S. action was hardly ethical. The need for an isthmian canal had been proposed by Alfred Thayer Mahan as a means of keeping the United States as a viable world power. Although Nicaragua offered better geography for the Canal, politics prevailed, and the Panamanian site was chosen. When Colombia (of which Panama was a part at the time) held out for a larger payment than the United States was willing to pay, the U.S. supported a rebellion in Panama, sent warships to prevent Colombia from landing troops to put down the rebellion, and immediately recognized the independence of Panama in exchange for the right to complete the canal begun by the French. This could hardly be described as ethical.
This is, of course, a matter of opinion, but I would have to say that it was in no way ethical.
First of all, the US tried to negotiate with Colombia to get the canal zone (Panama was part of Colombia at the time). When that failed, the US helped Panamanian rebels rebel against Colombia. In other words, the US helped break another country apart for the US's own benefit.
Second, once Panama became independent, the US did not even negotiate with actual Panamanians. Instead, they negotiated with a Frenchman and came to a deal with him before the Panamanians got to Washington. The Frenchman had a financial interest in building the canal in Panama so he could hardly have been negotiating only in the interests of Panama.
One can certainly argue that these things together make the US's actions in this case less than ethical.