The best replacement for treaty is executive agreement.

The best alternatives to treaty are rated as following:
★★★★★  executive agreement
★★★★  constitution
★★★★  convention
★★★  facultative reinsurance
★★  law
★★  agreement
★  versailles
★  facultative
★  auction
★  statute
★  us constitution
★  treatise
★  memorandum
  covenant
  declaration
  truce
  armistice
  conventions
  2nd amendment
  an executive agreement

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Speaking about treaty, one probably shouldn't forget about reservations.

Other search terms related to treaty and their ratings are:
★★★★★ reservations
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