of conservative policies. A poll taken in March 1987 showed that 85 percent of respondents believed that the Reagan administration had engaged in an organized cover-up, and half of the respondents believed that Reagan had been personally involved. The administration's credibility was also badly damaged on the international stage, as it had violated its own arms embargo on Iran. Congressional Democrats considered impeaching, but decided that it would be an unwise use of political capital against a weakened president; Democrats were also somewhat mollified by Reagan's decision to replace Chief of Staff Regan with Howard Baker. The investigations into the Iran–Contra scandal continued after Reagan left office, but were effectively halted when President George H. W. Bush pardoned Secretary of Defense Caspar Weinberger before his trial began. Investigators did not find conclusive proof that Reagan had known about the aid provided to the Contras, but Walsh's report noted that Reagan had "created the conditions which made possible the crimes committed by others" and had "knowingly participated or acquiesced in covering up the scandal." End of the Cold War Three different Soviet leaders died between 1982 and 1985, leaving the Soviets with an unstable leadership until Mikhail Gorbachev came to power in 1985. Although the Soviet Union had not accelerated military spending during Reagan's military buildup, their large military expenses, in combination with collectivized agriculture and inefficient planned manufacturing, were a heavy burden for the Soviet economy. Gorbachev was less ideologically rigid than his predecessors, and he believed that the Soviet Union urgently needed economic and political reforms. In 1986, he introduced his twin reforms of perestroika and glasnost, which would change the political and economic conditions of the Soviet Union. Seeking to reduce military expenditures and minimize the possibility of nuclear war, he also sought to re-open negotiations with the United States over arms control. As his influence on domestic affairs waned during his second term, Reagan increasingly focused on relations with the Soviet Union. Reagan recognized the change in the direction of the Soviet leadership under Gorbachev, and shifted to diplomacy, with a view to encourage the Soviet leader to pursue substantial arms agreements. Reagan's personal mission was to achieve a world free of nuclear weapons, which according to Jack F. Matlock Jr., Reagan's ambassador to Moscow, he regarded as "totally irrational, totally inhumane, good for nothing but killing, possibly destructive of life on earth and civilization." Gorbachev and