The cited text isn't syntactically valid - it's missing the preposition of...
The Americans' fear of China's military leaders — untested in combat — combined with an array of new high-technology weaponry is increasing the danger that [blah blah] will happen.
That's to say the grammatical "subject" of the sentence is a "compound noun phrase" - X combined with Y (where X = the Americans' fear and Y = the new weaponry).
Regarding the "plurality" of that subject, consider...
Oil combined with vinegar makes a quick and easy salad dressing.
No native speaker would treat the subject (oil combined with vinegar) as a plural, so we would never use make there. You could rationalise that by thinking of the subject as a (syntactically singular) combination, but it's probably more helpful to see combined with vinegar as just an "adjectival complement" (same as, say, oil pressed from olives). In strict syntactic terms, the underlying subject is simply singular oil, regardless of whether that subject is modified by a clause linking it to some other noun.
If anyone really cares, I should point out that even though logically it seems pretty likely that parenthetical untested in combat refers to China's military leaders, syntactically it could equally well refer to the Americans' fear [of those Chinese military leaders].
EDIT: Noting that several other answers and comments claim the cited text is "poorly written", I'd just like to say I completely disagree. Having just skimmed over the entire article, I think it's obvious the writer is very competent (one missing preposition is just a proofreading error, not a sign of poor writing skills).
Bear in mind the text is clearly aimed at competent readers, not inexperienced non-native speakers. The writing style may be relatively "dense" and "erudite", but it's certainly not "defective".