Panama is easy...
A series of war games by the USN hilight the vulnerability of the Panama locks to bombs, shells, or sabotage in 1926-1927. Add on to that, the possibility of wanting to build battleships wider than 108 feet, and the amazing cost of building wider locks, and Panama is seen as vulnerable.
So, the USA invokes its right to build a sea level canal through Nicaragua, and work starts in 1929. Work slows in the depression, but still progresses. The canal is finished in 1940, just in time for the war.
After the war, the Panama Canal is redundant compared to the wider, and more easily widened even further, Nicaragua Canal, so it might be let go.