The big question of The Handmaid's Tale, other than "will Offred ever escape to Canada," is "how did this happen?" How did Gilead rise? Yes, fans know Serena Joy wrote a book, The Sons of Jacob staged some terror attacks, and the American people didn't ask questions. But why were they so willing to accept this theocratic government that told them sin had brought about America's fall? The answer is The Infertility Crisis, which made people feel like they were being punished. But why is there an infertility crisis in Gilead in the first place?
This is, quite possibly, the hardest question the show has to answer, and so far, they've less answered it directly than say what it isn't. That's because the book doesn't answer the question, either. It doesn't have to. There's a fertility crisis, Offred is one of the few able to have babies, and that's the way the world just is.
But like a lot of the on-page details, there are suggestions something much larger happened in the recent past, which no one discusses. Those who are sent "to the Colonies" are cleaning up nuclear radioactive material from the heartland of America. It also indicates Gilead is actually much smaller than America was because the middle of the country experienced nuclear devastation at some point and is unlivable. That nuclear devastation is never directly said to have caused the infertility crisis. But at some point, a bomb was dropped, there was radiation, and now people can't have kids. You do the math.
This season, the show introduced the Colonies and those sent there to clean it up. Yet, they never actually talk about a nuclear attack, or why this part of the country looks like a nuclear fallout zone.
This is partly because the show has competing impulses. On the one hand, they're building the world of the book, where the heartland of America is a giant rift of nuclear waste dividing the two coasts. On the other, they are trying desperately to make this show as close to reality as possible in order to break through any denial of "it can't happen here."
The novel came out in 1985, during the height of the Cold War. It was fine to vaguely suggest an alternate near-history where a nuclear bomb dropped during the 1960s, and as a result America had been infertile for a decade and change because of the fallout, and now in the 1980s, a theocracy had risen. With Reagan's courting of evangelicals in the 1980 election, the "what if" part was still very close to home.
But the Cold War ended. No one bombed Kansas in the mid-1990s, and to suggest an alternate history where that happened is a much larger stretch. So instead, the show just vaguely handwaves about a fertility crisis, vaguely hinting it's been happening since circa ~2000 or so.
What's interesting is that disconnecting it from being due to nuclear fallout makes it more of a "mysterious medical crisis." This infertility could have come from some mutation of a virus. It also means the show can play around with the details.
In the books, the crisis is everyone. Men and women are infertile. The few women who can have babies, like June, are therefore a precious resource. The show changed that up. In the Canada episode, Mark, the American diplomat, says softly to Serena Joy that they've made headway in their research. It's not women who are infertile. Serena Joy may be infertile because she took a bullet to the ovary, but in reality, most women are just fine. A small surgery could give her a child of her own.
It's the men who have been rendered infertile. Gilead and the religious fundamentalists are just blaming women because they can't handle hearing their manhood is compromised. It makes for an interesting twist, and one the show can explore further in Season 3 come next year.