This past year’s insurgent uprising of overtly political films — “Good Night, and Good Luck,” “Syriana,” “Munich,” and “The Constant Gardener,” to name the most prominent handful — raises a question as old as cinema. What is a political film? And is a movie only as political as it is effective?

If the latter is the case, then the most political work of cinema in the last 20 years is Errol Morris’ “The Thin Blue Line” (1988), which actually proved the innocence of a man on death row — while, coincidentally, taking apart the criminal justice system in Texas.

Conversely, Michael Moore’s “Fahrenheit 9/11” (2004), which wanted to stop George W. Bush from being re-elected president, might be considered an abject failure — this despite a relatively low-budget and a gross of nearly $120 million, a blockbuster by documentary standards.

Accomplishment can be a slippery thing to grasp or define. Ask people who were going to the movies in 1962 and it’s likely that they’ll tell you the most socially significant film of their lifetime was “To Kill a Mockingbird,” based on the Harper Lee novel about a black man falsely accused of raping a white woman.

As U. of Alabama professor Claudia Durst Johnson has pointed out in her “Understanding ‘To Kill a Mockingbird,’ ” the story has obvious parallels to the infamous Scottsboro Boys case, which gave it added gravitas. So did the timing of its release, at a crucial moment in the civil rights struggle.

Whether Robert Mulligan’s film (and Gregory Peck’s mythic performance as Atticus Finch) broke down a nation’s racial biases in any way cannot be measured scientifically. But popular opinion would say it went a long way toward doing so — if not quite far enough: The just-concluded Sundance Film Festival featured a wonderful/horrible documentary entitled “The Trials of Darryl Hunt,” which told a similarly sad story all over again.

Political complexion can change according to circumstances: The original “Manchurian Candidate,” one of the most famously political movies of the last half-century, began life as a thriller that reflected the nation’s Cold War paranoia.

After the death of President John F. Kennedy, the film’s use of political assassination turned it into something else. Accounts vary, but Frank Sinatra, a one-time Kennedy confidante, is said to have had the film pulled because Lee Harvey Oswald allegedly watched it before killing JFK. Others say the film had simply run out of box office gas. Either way, Sinatra bought the film in 1972, and it subsequently wasn’t seen for years.

Rewinding to cinema’s infancy, “Birth of a Nation,” the landmark D.W. Griffith Civil War/Reconstruction epic, has become an increasingly hot-button movie thanks to its combination of cinematic virtuosity and Griffith’s virulent yet almost casually racist view of blacks in his native South.

Like the anti-Semitism of certain 20th century poets, Griffith’s poisonous racial point of view has long threatened his stature as an innovator and raised the question of whether content and art are mutually exclusive — which is itself a political issue.

There have been politically minded movies every year, some of which have even won best picture Oscars — “Casablanca” in 1942, for instance, or “Gandhi” in 1982. Others have been nominated — “Z” in 1969, “All the President’s Men” in 1976 and “Reds” in 1981. Some deserved to make the cut but didn’t — “The Candidate” (1972), “Wag the Dog” (1997) and “Hotel Rwanda” (2004).

That politics and film always skew left is a political truism rooted in the perceived liberal proclivities of Hollywood, and filmmakers in general (see Warren Beatty’s “Bulworth,” Rob Reiner’s “The American President” and Oliver Stone’s “JFK”). But what defines a political film is often in the eye of the beholder.

This year, the Christian right took “March of the Penguins” under its wing, perceiving the $77-million-grossing documentary as promoting fidelity, family and moral fiber (even though a spokesman at the American Museum of Natural History said the emperor penguins’ single-season marriages really classified the birds as “swingers”). And elsewhere in the animal kingdom, doesn’t “King Kong” contain a comment on capitalism and cutthroat government?

In the 1933 Merian C. Cooper/Ernest B. Schoedsack original, the authorities are very careful not to shoot at the big guy while he’s rampaging around Manhattan — not, at least, while he’s carrying the lovely Ann Darrow in his hand. In the Peter Jackson remake, the police and military couldn’t care less about endangering Naomi Watts’ character — they fire away recklessly and constantly, and seem to be far more interested in defending real estate than protecting one young woman’s life.

If that isn’t political, it’s hard to say what is. Or to argue that you can’t find movie politics everywhere you look.