> What are the alternatives besides something oppressive like using military power to prevent other countries from taxing their citizens too little?

Dropping the dogma of free trade and free flow of capital, and taxing any transactions involving tax havens to the point where it's not profitable to use them for tax avoidance.

You can only have countries' tax systems and regulations competing in global "free market" of jurisdictions if the countries are actually free to set arbitrary tax rates and tariffs.