What did washington mean when he wrote that the maxim "honesty is the best policy" should be applied to foreign relations?

There was no need to be coy about U.S. intentions to avoid permanent alliances in the future.

This proverb in international relations has a great importance. If honesty is present in relations then chances of conflicts are lesser. The same statement applies in terms of making treaties and pacts with other countries. The maxim used there is called Pacta Sunt Servanda which means “treaties shall be complied with". According to pacta sunt servanda all the states should show honesty while making treaties to maintain global peace.

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