I don't think I've ever articulated that but I'll give it a try.
Let's look at practical observation first, independent of theory: If you take a series of similar events in, say, U.S. history and compare it to the primary U.S. horoscope (such as the start of war, the end of war, economic collapse, murder or other death of presidents), you can indeed see these in the transits, progressions, and Sidereal solar and lunar returns of the U.S. chart. (First: This requires we settle on the best U.S. chart. The one for July 4, 1776 that clearly stands head and shoulders above all the others is the Hazelrigg-George chart for 12:15 PM.) The events show with clear enough indicators. However, they show even more easily, even more clearly with the Sidereal ingresses.
No single example ever proves anything, but a pattern of many examples will show a really dramatic trend. For example, transits to the Declaration of Independence chart for 9/11 had one aspect, transiting Pluto opposite natal Uranus within 1°. That's descriptive of the event and the impact, but quite general. You do have to dig into other layers, other predictive techniques (and also look at it across the range of time) in order to get the full showing in the U.S. nativity. -- However, in contrast look at my analysis of the same event in Chapter 23 of
Sidereal Mundane Astrology and you will see that it lays out in excruciating detail for Washington and NYC. (Just one of dozens to hundreds of possible examples.)
A digression: There is a characteristic of using such birth charts of nations that is sometimes an advantage, sometimes a disadvantage. It is that there are often multiple conceivable birth charts for different layers of the "birth" of a country. For example, the Declaration of Independence chart seems the central chart for the U.S. and especially describes the heart and soul of the nation, it's character, and most of the primary events. But the July 6, 1775 chart for the original declaration of war on England seems especially to describe our wars and conflict. The chart for the adoption of the Constitution seems to respond for events involving the structure and operation of the government (and many find so useful as to consider it the best U.S. chart overall). The chart for the first opening of the U.S. Supreme Court has been used by some to monitor trends in the court. And each presidential administration has a chart for its commencement that has been highly responsive for more major events of that administration than not.
This is an advantage to astrologers willing to work those individual charts in detail, study them separately, and distinguish which slices of the nation are most likely affected by major developments in the core U.S. chart. It's a disadvantage, though, not only because it requires an enormous amount of additional work, but because it gives more opportunity to opportunistically "find" an astrological pattern that confirms your biases. (
More data isn't always useful. Sometimes it's just white noise that lets you prove anything you want.)
Returning to the original question: The ingresses are independent of any construct such as a nation or a government. They show the primary psychological (including "event") forces on a specific geographic location at a specific point in time. We have learned from thousands of charts that when the location is a national capital, that location acts as a "hub" of the nation - the charts for Washington, DC affect the United States as a whole, those for Moscow affect Russia as a whole, etc. (See the brief Chapter 6 in my book.)
With astonishing accuracy (certainly when compared to any other astrological technique), Sidereal ingresses have shown the ability to discern the primary energies operating at a specific place and at a specific time. Faithful description of the
nature of an event while identifying a
specific time and
specific place is the reasonable basis for saying the technique showed the event. When forecasts for nations are concerned, "specific place" often
starts as Washington and then also shows for a place in the country where it occurred. (Look, say, at the Columbine shooting to see, first, the event itself in Columbine and the impact on the community in the aftermath; then, separately, the impact on the nation as a whole from the same charts for Washington.)
I don't know that the book is the answer to the world's troubles, but it is the answer to your original question about how we do mundane astrology here. It is the only book existing that describes how to use these tools (which correctly describe major events over 90% of the time).