"The hardest song to write is a protest song," writes James Sullivan in his new book Which Side Are You On? He's describing the view of Joan Baez, although, as Sullivan notes, "few of her contemporaries seemed to think so."

Baez certainly appreciated the artful constructions of her onetime protege Bob Dylan. So have generations of listeners across a wide range of demographic groups, but Sullivan reports that when Sam Cooke heard "Blowin' in the Wind," he thought African-Americans should be contributing their own Civil Rights anthems. "Cooke," writes Sullivan, "marveling that it took a white boy to write it, vowed to write his own song for the progressive movement."

The result was the song that's become regarded as the most significant of Cooke's career: "A Change Is Gonna Come." The version released to radio, though, was bowdlerized: the verse explicitly referring to racism ("I go to the movie and I go downtown/ Somebody keep telling me, don't hang around") was omitted.

When you think of 20th century protest songs, the Civil Rights Movement is probably what springs most immediately to mind. In addition to the aforementioned songs, the struggle against deadly violence and institutionalized oppression also produced Nina Simone's "Mississippi Goddam," Billie Holiday's devastating rendition of "Strange Fruit," and the modern version of "We Shall Overcome," a song with origins in traditional gospel music.

All of those songs are described in Sullivan's chapter on Civil Rights, as is "Keep On Pushing," a 1964 song by the Impressions featuring a young Curtis Mayfield. After Martin Luther King Jr. was nominated for the Nobel Prize, Mayfield didn't find the lyrics difficult to write at all. When his bandmate asked what inspired the words, Mayfield simply responded, "I'm living."

One of Sullivan's challenges is that it turns out to be surprisingly difficult to define what a "protest song" is. His chapter titled "No Nukes" mentions P.F. Sloan's "Eve of Destruction"; popularized by Barry McGuire, the song is unmistakably a cautionary ballad about nuclear war. Does Prince's "1999" really belong in that chapter, though? Particularly given that the book makes no mention of "Sign O' the Times," the song that many Prince fans would consider his greatest song of protest?

A much lesser-known Prince song, the pro-vegan "Animal Kingdom," would also fit well in the chapter on songs about the environment. That may be the chapter with the most surprises for music heads. Did you know, for example, that the Beach Boys released an anti-pollution song? ("Don't Go Near the Water," the opening track on 1971's Surf's Up.)

Or that a 1970 record of whale "songs" helped inspire a major environmental movement? In what may have become the most famous quote by a recording engineer since Bob Johnston's "Is it rolling, Bob?", the two biologists who analyzed the contents of Songs of the Humpback Whale received the recordings from a Navy technician who said, "Go save the whales."

Is it a stretch to name the album's opening track, "Solo Whale," one of the most important 100 protest songs of the 20th century? Sure, but...why not.

In other categories, protest songs prove even more difficult to define. Sullivan has a chapter on songs advocating freedom of speech, and certainly Crosby, Stills, Nash & Young's "Ohio" belongs there. Nirvana's "Smells Like Teen Spirit," though? Sullivan sees it as a "complex critique of middle-class complacency."

The chapter on gay rights is particularly poignant, because it was only near the century's end that songs about gay rights could be explicit. For most of the century, queer activists essentially had to appropriate tracks like Connie Francis's "Where the Boys Are" and the Youngbloods' "Get Together."

While Nile Rodgers and Bernard Edwards of Chic wrote "I'm Coming Out" for Diana Ross as a deliberate statement of gay pride, they just as deliberately kept the lyrics ambiguous enough that the song didn't have to be interpreted that way. "You're leaving Motown, changing your life," Rodgers told Ross, who understandably thought the song might be understood to be about her own sexuality. "You're an independent woman. You're coming out of your shell."

Independent women, happily, get their own chapter, and it's country artists who garner much of Sullivan's attention. Dolly Parton's "Just Because I'm a Woman" (1968) criticized the double standard, and Loretta Lynn's "The Pill" (1975) was a crucial conversation-starter at a time when birth control was still taboo in many country circles.

Soul artists (Aretha, "Respect") and blues pioneers also get their due in that chapter, but hip-hop gets surprisingly short shrift. A few pages in the final chapter, on 21st century protest songs, run quickly through Grandmaster Flash, Public Enemy, N.W.A., and Kendrick Lamar, but that doesn't feel like nearly enough.

It's hard to argue with the way the book ends, though: with "Immigrants (We Get the Job Done)," from Lin-Manuel Miranda's Hamilton. An earlier chapter on "Immigration and the 'Other'" opens with the "Plane Wreck at Los Gatos (Deportee)," Woody Guthrie's last major song. It was inspired by a 1948 California crash that took the lives of 28 Mexican citizens, including both temporary workers and undocumented immigrants.

Who are all these friends
Who are scattered like dry leaves
The radio said
They were just "deportees."