Let’s take a close look at the usage of the conjunction “which” in the sentence you quoted:
“Looked at from another angle, the aim of the ‘Long March to resist Japan in north China’ was to allow CCP members to rest and recoup their strength, and provided a reliably safe and self-sufficient base area in the north-west for the armed resistance against Japan which came later.”
In that sentence, “which” is used as a function word to introduce a subordinate clause expressing consequence, result, or effect. That subordinate clause is “which came later,” which is a restrictive clause—meaning that this clause is essential or indispensable to the meaning of the sentence. (The opposite of a restrictive clause is, of course, the nonrestrictive or nonessential clause—meaning that the clause is not absolutely necessary to the meaning of the sentence. Put in another way, a clause that follows “which” merely adds information to the sentence and can actually be taken out without altering the basic idea.)
Now, in American English, the convention is to use the function word “that” to introduce a restrictive or essential clause, and to use “which” preceded by a comma to introduce a nonrestrictive or nonessential clause. This American English convention is the usage that you are particularly referring to, and you are absolutely right in expecting that comma to precede the “which” in that sentence, in which case it should read as follows:
“Looked at from another angle, the aim of the ‘Long March to resist Japan in north China’ was to allow CCP members to rest and recoup their strength, and provided a reliably safe and self-sufficient base area in the north-west for the armed resistance against Japan, which came later.”
But the big question is: Why wasn’t that comma provided before the “which”?
The answer is that either that sentence is grammatically wrong or that it’s not in American English to begin with. It was therefore necessary to check the provenance of that sentence. When I did, I found out that it was apparently written by Xuē Xīnrán, pen name Xinran, who is a British-Chinese journalist, broadcaster, and writer. That usage of “which” is therefore grammatically correct because the sentence you quoted is in British English, in which the grammatical convention for the usage of “which” is different from that of American English.
In British English, “which” is used in place of “that” for introducing both restrictive and nonrestrictive clauses. The British English convention marks a nonrestrictive clause by preceding the “which” with a comma; when the clause is intended to be restrictive, no comma is used to precede “which.” In the case of the sentence you quoted, the absence of that comma before “which” is a telltale sign that we have a British English sentence here that an American English writer would have rendered with a “that”—with no comma before it, of course—as follows:
“Looked at from another angle, the aim of the ‘Long March to resist Japan in north China’ was to allow CCP members to rest and recoup their strength, and provided a reliably safe and self-sufficient base area in the north-west for the armed resistance against Japan that came later.”