I think/would hope that the documentary's goal was to emphasize that diplomacy was a key element in Inca expansion. However, this was paired with their growing reputation as one of the scrappiest and hard-fighting military machines of the Andes. So quite briefly, the Inca empire was due to astute political and military application.

Let me give you an anecdotal idea (supported by chronicler accounts) of what a "typical" Inca expansionary move would look like. And because it's Friday, I'll make it a little dramatic narrative. (I do this in lectures, it's quite fun.)

Joe Q. Farmerman: Boy, what a gorgeous day in my valley community. The harvest has never been better!

Ike Inca: Ho there, what a fine harvest indeed!

Joe: How did you get into my home?

Ike: Say, you're the community leader here. You have a modest surplus, and this valley is one of the most fertile in the Andes! I represent the Inca, the children of the sun god Inti. Your land - well, I hate to say it, but it's really my lord's. See, the Inca are the chosen people of Viracocha, our common creator god. You may know him by another name, but that's not important. We can all work together! Under the Inca banner. You will retain all your prestige and community standing, if you cooperate.

Joe: Seems nice, but what if I refuse?

Ike: Heh, I always get people who at least ask. Well, I didn't come alone on this diplomatic envoy - matter of fact, there are roughly ten thousand Inca warriors on the other side of that mountain over there, about a day's march away. Remember Siquillapucara?

Joe: No...

Ike: Exactly. So what do you say? We will be here in the morning. Will we be greeted as liberators? Or as conquerors?

And scene.

As you can imagine, most every small polity the Inca encountered at the height of their expansion took the "not dying" option. Now, this isn't the only way the Inca expanded, mind you. The early conquests in their immediate hinterland were much more "loot and pillage" affairs for the benefit of the Inca upper class - as D'Altroy (2002: 83) puts it, "combining predation and self-defense." The Inca were not always the big bullies on the block. As they left their home valley, to the south were the Qolla and Lupaqa kingdoms, already well-fortified from decades of warfare (a time period known by Andean archaeologists as the Late Intermediate Period - but far from a boring time!). To their northwest were many groups, most notably the Chimu, who had control over hundreds of miles of coastline and had constructed the largest adobe-brick city in the world. The Inca, to put it mildly, were dark horses to control the Andes.

And this is where the emphasis on diplomacy really comes in, which (without having seen this documentary) I'm guessing is the BBC's intent. The Inca brought the Qolla and Lupaqa into the fold by allying the Lupaqa and turning them on the Qolla and Ayaviri. Cieza (a chronicler) held that during this campaign two Chanka lords were heading up this expeditionary force - the Chanka were northern nemeses of the Inca and one of their first vassals. With these groups subdued Pachakuti (the first expansionist Inca emperor) turned northward, to Cajamarca and later the Chimu on the coast. However, this was all done with an agglomerate of Inca generals leading various ethnic groups' armies and trying to keep them on target for the ambitions of Cuzco. Alliances were secured with the northern extent of the empire based out of Quito, mostly by way of securing royal marriages with the local elites. These northern lords maintained a measure of autonomy and contributed much of the military prowess of the later Inca forces - indeed these northern armies eventually wrested control from the Inca Huascar under the banner of Atahualpa, Huascar's half-brother and a prince of the northern realms.

I will give this documentary a watch though, I'm sure it's a good program. It's the BBC we're talking about, not H2.