Pfizer and Allergan said they had terminated their $152 billion merger just days after the Obama administration introduced new rules meant to limit the ability of American companies to shift their home overseas simply to lower their taxes.

The deal, announced last November, would have been the largest transaction of its kind, a so-called “inversion” that allows an American company to shed its United States corporate citizenship in order to move income beyond the reach of American tax authroities. Allergan has its tax domicile in Ireland.