“Strong men can seal a deal with a handshake with no thought of it ever being broken.”
I would not characterize either Hitler or Stalin as “weak”. Ambitious, amoral, charismatic, evil.. but not weak.
Political leaders break promises when it is to their advantage to do so. Machiavelli had it right on this.
Reagan had it right also, when he said “Trust, but verify.”
Sometimes the physically strong are internally weak and use their physical strength to compensate for or mask their inner weakness and/or to service their ambition rather than the needs of those they lead.
These types will demonstrate how "a chain is no stronger than its weakest link" in the right/wrong circumstances.
I see Hitler as such a person and would characterize him as weak: no integrity, no virtue, no morals, no heart, just ego and the results to match.
Perhaps Stalin was not too different and the Russians suffered his weaknesses under the Soviet heel which Stalin's weakness made necessary. Were he not weak, no jack booted thugs would be needed.
North Korea has a similar sort of problem.
Regarding such people, Reagan had it right, but I also like quoting Ordell Robbie from the movie Jackie Brown:
"You can't trust Melanie, but you can trust Melanie to be Melanie."