The Chafetz Chaim on Torah Study vs. Hatzalas Nefashos
Sefer Toras HaBayis (by the Chafetz Chaim), 5th Perek:
"אבל באפשר המצוה לעשות ע"י אחרים, גדולה היא התורה מכל המצות, ואף בענין פקוח נפשות, אם נזדמן לאחד להציל נפשות, ולשני לא נזדמן זה, ורק עסק לו בתורה, וישאלו אותנו מי הפעיל יותר, לכאורה היו הכל משיבין כי המציל נפשות הגדיל לעשות, אבל חז"ל גילו לנו שאף שגדול מצותה של פיקוח נפש מאוד, אבל זה שלא נזדמן לו מצוה זו, ורק עסק בתורה באותו זמן לא נופל ממנו, ואדרבה, הוא הגדיל לעשות, שגדול ת"ת מהצלת נפשות" - תורת הבית פרק ה
"However, when the Mitzvah can be done by others, Torah is the greatest of all the Mitzvos, and even in the matter of preserving life, if one person had an opportunity to save lives, and the other did not have this opportunity, and they will ask us - "Who accomplished more?", we would, seemingly, all respond that the one who saved lives accomplished more. But Chazal revealed to us that although the Mitzvah of Pikuach Nefesh is very great, but the one who did not have the opportunity to fulfill this Mitzvah, and he only learned Torah at that time, is not inferior to him, and on the contrary, he has accomplished more, for the learning of Torah is greater than the saving of lives."
Yes, this is meant as a response to all those who call those who learn Torah "parasites". The Chafetz Chaim is telling us that when the Mitzvah to save others can be done by others, it is those who are learning who are accomplishing more. Of course, if it is necessary, we have the rule of "Afilu Chassan MeChadro V'Kallah MeChupasah" for a Milchemes Mitzvah. Nobody is exempt at a time of obligatory war, when they are needed. But, just like not every single man, woman, and child is in army fatigues at this moment, because the army has determined that they are not needed at present, so too the army and the Israeli legal system have determined that those who are learning are not needed for the war effort at present, and therefore they are free to pursue the greater accomplishment of Torah study. (Which serves as a protection to Klal Yisrael in its own right. See: Pesachim 87a with Rashi; Tanna D'bei Eliyahu Rabbah 21; Eichah Rabbah Pesichta 2; Yeshayahu 62:6; Tanna D'bei Eliyahu 10)
However, those who are learning, and especially those who are neither fighting nor learning, and this is directed at myself as much as anyone, would do well to bear in mind the next part that appears in the Toras HaBayis:
"And see now, my brother, that according to this, for each and every hour that you could have learned Torah, and you did not, it is as if they came before you to save lives, and you refrained from saving them."
And he adds:
"And although us, with our sealed eyes, would not know this, but Chazal revealed this secret to us, how great is the value of one who learns Torah, that there is nothing in the world that is equal to it."