Alliances as a means, not an end

Loyalty and faithfulness to commitments are good things, but nations aren't people. George Washington warned us that

“habitual hatred or a habitual fondness” turns a nation into “a slave to its animosity or to its affection, either of which is sufficient to lead it astray from its duty and its interest.”

That's not to say that a nation should lead other nations on, then disappoint them, only that it should think carefully about what it commits to.  In other words, exhibit a little impulse control.