U.S. president-elect Donald Trump threatens to bring about a major shake-up, questioning the U.S.'s role in NATO, relations with China, Iran and opening the way for more cooperation with Russia in a series of often aggressive rhetoric. Trump marks a sharp shift in U.S. national security policy and a jarring departure from the stance of President Barack ObamaTrump challenged NATO during the presidential campaign, suggesting that the U.S. might not defend partners that don't fulfill financial obligations to the longstanding U.S.-European military alliance.Trump shouted loud and clear over the summer that the United States stands ready to help the European nations in the face of a Russian threat only if they pay their fair share for NATO's combined defense. It called into question the vaunted Article 5 of the alliance treaty, which states that an attack on one member is an attack on all, and that the target would automatically receive help and assistance.Considering the relations with Russia, Trump, who has repeatedly praised Putin and nominated people seen as friendly toward Moscow to senior administration posts, signals a new ear for U.S.-Russia ties before taking office on Jan. 20.In late December, Russian President Vladimir Putin and Trump appeared to unexpectedly announce an intensified nuclear arms race. It was, perhaps, an early sign that relations between the U.S. and Russian leaders may not be as positive as some had expected, as Reuters news agency reported.The U.S. and Russia hold the vast majority of the world's nuclear weapons. In 2010, the two countries signed the New START treaty capping the number of nuclear warheads and missile launchers each country can possess. The agreement is in effect until 2021 and can be extended for another five years.The diplomatic thaw between Washington and Tehran over the past two years looks in jeopardy with U.S. President-elect Donald Trump taking office. He said during his election campaign that he would scrap the nuclear agreement. The agreement, signed in Vienna in July 2015 and in force since January, was the signature diplomatic breakthrough of Barack Obama's second term. It calls on Tehran to curb its nuclear program in exchange for sanctions relief from the U.S. and other nations. Trump has promised to tear up the nuclear deal once in office, calling the agreement under which it was implemented -- the Joint Comprehensive Plan of Action -- the "worst deal ever negotiated."As Trump threatens to abandon the nuclear treaty with Iran, China reacted to the implementation of the Iran nuclear deal, saying it should not be "affected by any changes in the domestic situations" of the countries involved.Trump angered China when he spoke to Tsai last month in a break with decades of precedent and cast doubt on his incoming administration's commitment to Beijing's "one China" policy. The United States, which switched diplomatic recognition from Taiwan to China in 1979, has acknowledged the Chinese position that there is only "one China" and that Taiwan is part of it.