Here's a look back at the five moments that stand out. (Did we miss any? The comments section awaits!)
1) "The second most important man in my life." Bound by opposition to communism, Thatcher and Reagan shared a close bond throughout the 1980s. Together, they provided a united western counterbalance against the Soviet Union, and modernized Europe’s anti-Soviet nuclear shield. Thatcher once referred to Reagan as the "second most important man" in her life.
2) Strains in the relationship: The Reagan-Thatcher relationship wasn't always so rosy. Reagan didn't immediately support Britain in its conflict with Argentina in the Falkland Islands in 1982, urging the European ally to pursue talks. And Thatcher's government denounced the Reagan-sanctioned invasion of Grenada in 1983.
Documents that were newly declassified in 2012 revealed some of the details of the strains in the Thatcher-Reagan relationship. Thatcher once described a Reagan dispatch on the Falklands invasion as "so vague, I didn't think it was worth reading":
3) Address before a joint session of Congress: Thatcher addressed the U.S. Congress in 1985, winning rousing applause for a speech in which she vouched for the Reagan administration's foreign policies. "In an address that stirred applause in the packed House chamber -- especially among Republicans -- Mrs. Thatcher also stressed that she firmly supported President Reagan's space-based missile defense research plan," The New York Times reported at the time.
Below is a sampling of her speech. (The Margaret Thatcher Foundation has a complete transcript of the remarks.)
4) "No time to go wobbly." Toward the end of her tenure as prime minister, Thatcher helped spur on President George H.W. Bush to intervene militarily in the Persian Gulf after then-Iraqi President Saddam Hussein invaded Kuwait. Thatcher famously declared to the U.S. president that "this was not time to go wobbly."
Here's how Thatcher described her initial thoughts about Hussein's invasion of Kuwait, in an interview with PBS's "Frontline": "I went out for a walk, always lovely in the mountains, and got things worked out in my mind, but it was perfectly clear, aggression must be stopped. That is the lesson of this century. And if an aggressor gets away with it, others will want to get away with it too, so he must be stopped, and turned back. You cannot gain from your aggression."
5) Spurning Sarah Palin: In 2011, the Guardian newspaper reported that Thatcher would not be meeting with Sarah Palin during Palin's trip to London. "Lady Thatcher will not be seeing Sarah Palin. That would be belittling for Margaret. Sarah Palin is nuts," the paper quoted one Thatcher ally as saying. The reported rejection sparked an outcry among conservatives in the United States, lending a great deal of attention to the non-meeting.
It also sparked dispute over whether Thatcher's inner circle was in fact rejecting Palin. Nile Gardner, a former aide to Thatcher who is a foreign affairs analyst and political commentator at the Daily Telegaph wrote at the time: "I have spoken to Lady Thatcher’s Private Office regarding the story, and they confirm that the attack on Sarah Palin definitely did not come from her office, and in no way reflects her views. As a former aide to Margaret Thatcher myself, I can attest that this kind of thinking is entirely alien to her, and that such remarks would never be made by her office."
Updated at 11:48 a.m. on 4/9