延伸開來 in this context is a somewhat vague connective, but one could guess the author meant something in the neighborhood of:
- "if this attack is allowed to continue"
- "if this attack is allowed to expand to other fronts"
- "if our argument about the attack is taken to its logical conclusion"
- "and this attack will continue"
- "and this attack will expand to other fronts"
My first impression favors the "if" readings, because I would write this differently if I were to emphasize the "and" readings. So if we take this vagueness into account, the shortest natural translation would be "by extension."
It is perhaps the easiest to parse the phrase as 延伸+開+來:
- 開 attached to a verb suggests a sense of "broadening" or "spreading," as in 打開 "open up (a book, a window)," 張開 "spread out (one's arms)," or 放開 "release (one's hands, a burden)." You could say 開 is made redundant by 延伸, but the construction just sounds unnatural without 開.
- 來 is difficult to translate exactly because it simply conveys direction, namely, "outward"; in this sense, it serves the same purpose as 開. 來 is not strictly necessary in the examples below, but I cannot think of a sentence that sounds natural with just 延伸開.
The same construction in different contexts:
- 一定要把重點從小節出區分開來 "we must separate out the focus from the fine details"
- 病毒從醫院裏擴散開來以後，一共感染了五十多人 "since the spread of the virus from the hospital, it affected north of fifty people"