Also during Calvo-Sotelo's tenure, Spain negotiated and accomplished its integration within NATO in May 1982. This move was met with the staunch opposition of the parliamentary left, headed by Felipe González' PSOE, which promised to hold a referendum on the issue if it ever came to government. But it also caused a rift between Calvo-Sotelo and former PM Adolfo Suárez, still a UCD MP, on the grounds that the incumbent Cabinet had not been duly informed of the possible consequences of an eventual Spanish entry into NATO, as well as the haste and rush with which the integration process was being carried out.