Our prayers are almost always requests for mercy, as the gemara states (Berakhot 20b), that prayer is "mercy". The natural world has its laws of nature, and the Torah has established laws of punishment "measure for measure", but in our prayers we ask that these laws be circumvented: We ask HaShem to send rain even if the forecast wouldn't predict it, or to be lenient with us even if we really did transgress.
Prayers for justice, on the other hand, are extremely rare. The gemara warns, "Anyone who asks the judgment of his fellow man, he is punished first!" (RH 15b.) Rav Nachman writes that such a prayer is usually "eaten up" by the side of evil. It generally does not stem from the uplifting, idealistic side of man that inspires our other prayers, but rather from the small-mindedness and vindictiveness that are the usual fare of the evil impulse.
One who would pray for judgment needs extraordinary qualities. First of all, he must have unblemished righteousness; otherwise he will be punished first. Second of all, his request for judgment must itself stem from a recognition that ultimately such judgment is necessary in order for kindness to reign. We find for example, that the blessing asking for judgment on the "minim" could only be composed by Shmuel HaKatan who was known for his extreme self-effacement (see Sanhedrin 11a) and lack of vindictiveness (see Avot 4:19); furthermore, it was only introduced when it was clear that it was an absolute necessity to save the prayer service from malicious informers (Berakhot 28b). (Rav Nachman explains that such a necessity generally arises when mercy is distorted in order to protect and nurture wickedness and cruelty. Judaism reconciles itself to the need to be "cruel to be kind" only with difficulty, when the world considers it "kind to be cruel".)
Rav Nachman states that when such an extraordinary individual does arise and confronts such an extraordinary situa- tion, he has immense power to subdue evil and to awaken to repentance those who have been caught in its grip. In fact, it is this exact trait that gives a person the ability to reprove others in an inspirational way that affirms their basic goodness (as we explained last week). Rav Nachman calls this a "voice" or a "song" which awakens the dormant good in wrongdoers and gives them a beautiful fragrance that nullifies the stench of sin.
Rav Nachman refers here, as he often does, to a "single, double, triple and quadruple song"; he explains that these four levels refer to different levels of Divine providence. The lowest level is completely according to natural law, without any Divine guidance (though of course the laws themselves are of Divine origin!); the highest level is completely according to Divine intervention, as the world will be guided in the time of the complete redemption.
We can explain that someone who has the most profound understanding of HaShem's ways is able to perceive that sin ultimately is also part of HaShem's plan. What is considered against HaShem's will at a lower level of providence is actually part of His greater blueprint at a higher level. A normal person is not capable of such a perspective; if you tell him that evil is part of G-d's plan, then he will feel no distress in the face of wickedness, whether his own or of others. If he understands that evil is against G-d's will, then he considers the sinner banished from G-d. Only a few, such as Moshe, are able to encompass all these songs; these individuals are able to fight evil with all their might, yet reprove wrongdoers with a perfect faith that they are still servants of G-d, involved in advancing His plan.
Rav Natan writes that one actual song that gives expression to this supernal song is the Song of the Sea. This song celebrates the judgment of Egypt. Normally this would be highly inappropriate; the Midrash states that the angels were forbidden to sing during the splitting of the sea (Yalkut Shimoni Beshalach). But Moshe, who led Israel in this song, had a perfect apprehension of how this judgment, with its awesome demonstration of HaShem's sovereignty and His election of Israel, was necessary for the establishment of G-d's kingdom among mankind. This song refers to natural phenomena; to G-d's judgment and retribution; and ultimately to the final redemption: "HaShem will reign for ever and ever". Thus it encompasses all of the four levels of song.
We explained above that prayers for judgment are generally acceptable only for truly extraordinary individuals in truly extraordinary circumstances. Yet there is an exception: Rosh HaShana, the Day of Judgment. On this day, all of us pray for a favorable judgment: while we make pleas for leniency, ultimately we ask judgment to be done. Rav Natan explains that this special quality extends to all the New Years mentioned in the mishna, including Tu BiShvat, the New Year for trees. It seems that on these days all Israel merit a bit of the spiritual might which makes such a prayer acceptable. And on Shabbat Shira, all of us participate in the public recitation of the Song of the Sea; evidently on this day all Israel merit a dim apprehension of the "four levels of song". Since these two qualities are intimately connected, it is natural that Shabbat Shira and Tu BiShevat are always in close proximity.