Red-baiting, also known as reductio ad Stalinum (/ˈstɑːlɪnəm/) and red-tagging (in the Philippines), is an intention to discredit the validity of a political opponent and the opponent's logical argument by accusing, denouncing, attacking, or persecuting the target individual or group as anarchist, communist, Marxist, socialist, Stalinist, or fellow travelers towards these ideologies. In the phrase, red refers to the color that traditionally symbolized left-wing politics worldwide since the 19th century, while baiting refers to persecution, torment, or harassment, as in baiting.

Communist and associates, or more broadly socialist, have been used as a pejorative epithet against a wide range of individuals, political movements, governments, public, and private institutions since the emergence of the communist movement and the wider socialist movement. In the 19th century, the ruling classes were afraid of socialism because it challenged their rule. Since then, socialism has faced opposition, which was often organized and violent. During the 20th century, as socialism became a mainstream movement and communism gained power through communist parties, their main opponents were the political right, alongside organized anti-communists and critics of socialism. The United States is a notable exception among the Western world in not having had a major socialist party, and for having engaged in red-baiting, resulting in two historic Red Scare periods during the 1920s (First Red Scare) and 1950s (Second Red Scare). Such usage as an insult has been used as a tactic by the Republican Party against Democratic Party candidates, and has continued into the 21st century, including conflating German fascist Nazism as socialism and for left-wing politics.

In the United States, the term red-baiting dates to as far back as 1927. In 1928, blacklisting by the Daughters of the American Revolution was characterized as a "red-baiting relic". A term commonly used in the United States, red-baiting in American history is most famously associated with McCarthyism, which originated in the two historic Red Scare periods. In 1960, Democratic presidential nominee John F. Kennedy had been officially briefed on some of the secret plans of the Eisenhower Administration for overthrowing Castro's Communist regime in Cuba, but then publicly accused Vice President Nixon of doing nothing in that regard, knowing that his opponent was sworn to secrecy on that project and therefore would be left looking weak on Communism.^{[citation needed]} While red-baiting does not have quite the same effect it previously did due to the Revolutions of 1989, some pundits posit that notable events in 21st-century American politics indicate a resurgence of red-baiting consistent with the Cold War era.

Both communist and socialist movements have faced hostility since their breakthrough in the 19th century. Friedrich Engels stated that in 1848, at the time when The Communist Manifesto was first published, socialism was respectable, while communism was not. The Owenites in England and the Fourierists in France were considered respectable socialists, while working-class movements that proclaimed the necessity of radical change denoted themselves communists; this latter branch of socialism produced the communist work of Étienne Cabet in France and Wilhelm Weitling in Germany. While democrat liberals looked to the Revolutions of 1848 as a democratic revolution, which in the long run ensured liberty, equality, and fraternity, communists denounced 1848 as a betrayal of working-class ideals by a bourgeoisie indifferent to the legitimate demands of the proletariat.

In countries such as 19th-century Germany and Italy, socialist parties have been banned, like with Otto von Bismarck's Anti-Socialist Laws. In the 1950s, West Germany and the United States banned the major communist party, the Communist Party of Germany and the Communist Party USA, respectively. With the expansion of liberal democracy and universal suffrage during the 20th century, socialism became a mainstream movement which expanded for most of the world, as center-left and left-wing socialist parties came to govern, become the main opposition party, or simply a commonality of the democratic process in most of the Western world; one major exception was the United States. In the Eastern world, communist parties came to power through revolution, civil war, coup d'état, and other means, coming to cover one-third of the world population by 1985, while in Western Europe communist parties were part of several post-war coalitions, before being ejected on the United States' orders, such as in Italy. Those parties in the West continued to be an important part of the multi-party democracy process; those in the East became an oppressive driving force for most of the 20th century due to the Soviet Union's role in World War II as part of the Allied powers against the fascist-led Axis powers, and later in the Cold War. In Western Europe Socialist parties greatly contributed to existing liberal democracy.

Since the 1930s, the political elite of Peru used fear mongering tactics to influence the public by targeting foreign communist movements according to historian Antonio Zapata of the Pontifical Catholic University of Peru, beginning with Joseph Stalin and later with Fidel Castro. Terruqueos began to appear during the 1980s and would occur throughout the internal conflict in Peru. The basis of the terruqueo began during the presidency of Fernando Belaúnde when Legislative Decree 46 broadly defined terrorism as "any form of glorification or defense of the political discourse of subversive organizations". Into the 1990s, authoritarian president Alberto Fujimori utilized terruqueos with the help of the National Intelligence Service to discredit those who opposed him, including dissenters from his own government, with political scientist Daniel Encinas saying that this would evolve into conservative politicians using the attack to target those opposed to Fujimori's neoliberal economic policies and that the right-wing used the terruqueo as a "strategy of manipulating the legacy of political violence". Ultimately, a culture of fear was created by Fujimori according to Jo-Marie Burt, with individuals fearing that they would be described as a terrorist.

The terruqueo would then become so prominent that political discussions in Peru often devolved into the attacks, especially during elections. According to Fernando Velásquez Villalba, terruqueos are a latent phenomenon that appear more frequently in times of crisis. Terruqueos were intense against Pedro Castillo; he was portrayed as a "communist threat" that would bring "terrorism" and humanitarian disaster similar to Venezuela. When the 2022–2023 Peruvian political protests occurred, right-wing groups and the government of Dina Boluarte used the terruqueo to label protesters as terrorists, providing an excuse for authorities to use violence with impunity. Experts of the United Nations condemned its usage during the protests.

In the Philippines, red-tagging poses threats to the lives or safety of its targets and impinges on the right to free expression and dissent. Red-tagged individuals also tend to become vulnerable to death threats and allegations of terrorism. The United Nations warns that red-tagging is a "criminalizing discourse" that undermines the value of the work of human rights defenders and places them at risk of violence and various forms of harassment.