On August 6, 1997, Steve Jobs announced that Microsoft made a major investment in Apple. Microsoft bought $150 million in non-voting Apple stock, promised to hold that stock for three years, and promised to support Microsoft Office on the Mac for at least five years. In exchange, Apple dropped its lawsuit over Windows ripping off its user interface and made Internet Explorer the default browser on the Mac. (There were a handful of other terms, including a collaboration on Java, but they aren't particularly substantial.)
All of this was pre-iPod and even pre-iMac. At the time, Apple was busily bleeding money and was on the verge of bankruptcy. Shortly after, Apple began the meteoric rise that has sustained it for decades. From 1998 on, Apple launched a string of successes: iMac, iBook, iPod, iTunes Store, and eventually iPhone and iPad. (To be fair, we got some flops too.)
When Jobs announced the Microsoft deal, the crowd at Macworld Expo freaked out. Jobs now-famously said, "We have to let go of a few notions here. We have to let go of the notion that for Apple to win, Microsoft needs to lose." What is less often quoted is the next thing Jobs said: "We have to embrace a notion that for Apple to win, Apple has to do a really good job. And if others are gonna help us, that's great, because we need all the help we can get. And if we screw up and we don't do a good job...it's our fault. ... If we want Microsoft Office on the Mac, we'd better treat the company that puts it out with a little bit of gratitude. We'd like their software."
Here's video of that fateful announcement: