"It" refers to single objects (that can be implied)

A student today noted that when he took English language classes in Korea, he was told that the word "it" can only refer to a singular object, and not a clause in a sentence. My initial reaction was, "No, 'it' can be used to refer to a clause in a sentence, because I just used 'it' right there to refer to the preceding clause." Then I thought about it some more, and came to this realization:

If a clause or sentence can be simplified to a description that is a single thing, then that thing can be referred to as "it." Take the following example:

"The invading army was pouring through the mountain pass, swords waving menacingly above the distorted faces of the screaming soldiers."

This sentence can be simplified to the simpler phrase:

"Screaming, sword-waving invading soldiers charged through the mountain pass."

Here, the importance to detail is omitted in favor of a less-colorful description of "just the facts." However, this is not the single item we were looking for. However, this simplified sentence can be altered thusly:

"We were being invaded via the mountain pass."

In this modification, the details of "screaming", "sword-waving", and "charging" are removed, to describe the implication of what those omitted words mean. However, this is not yet that single item we are seeking. We will change it once more:

"The invasion."

Now we refer to the whole action's implication as one "thing". Therefore, we can refer to "the invasion" as "it." Voila! Now we can realize why the following two sentences are not disconinuous with one another.

"The invading army was pouring through the mountain pass, swords waving menacingly above the distorted faces of the screaming soldiers. It was not a good way of launching his first day as General of the Sun Legion, but Randolph Wimplebottom the 17th felt that he was given an opportunity to prove his valor."