In the sicha of Parshas Naso, the Rebbe states as follows:

This is especially emphasized in this year — the year 5751 (1991) which stands for “I will show wonders”– beginning with the wonders that we already saw in actuality, revealed in the eyes of the whole world, in this year [referring to the miracles of the Gulf War].  That through [these events] the words of the Yalkut Shimoni midrash were fulfilled: “In the year that Melech Hamoshiach is revealed all the kings of the world will quarrel, the king of Persia [Iran/Iraq], the king of Arabia, and the Holy One, blessed be He, says to the Jewish people, “my children, do not be afraid, all that I have done I only did for you…the time of your Redemption has arrived.”  Since that time, we are already standing at [the closing section of the Yalkut Shimoni midrash] “in the hour that Melech Hamoshiach comes, he announces to the Jewish people and says humble ones, the time of your redemption has arrived.”

Prior to and during the first Persian Gulf War, the Rebbe made frequent references to this Yalkut Shimoni midrash, but here, for the first time, the Rebbe says unequivocally that “it has been fulfilled”, specifically quoting the words “in the year Melech Hamoshiach is revealed”.  In other words: in 1991 Melech Hamoshiach was revealed.  But despite this, as the Rebbe points out in other sichos, the Jewish people are still fearful and require Moshiach to tell them not to be afraid and to inform them that the Geulah has arrived.  But the “breakthrough” has happened: the year Melech Hamoshiach is revealed.

To add further insight:

“The year in which Melech Hamoshiach is revealed…” (“שנה שמלך המשיח נגלה בו”). The word “year” in Hebrew is feminine, and grammatically we would expect this Midrash to read “נגלה בה“, meaning “in her [the year, feminine]”.  So we need to explain why it says “בו” instead of “בה”: “the year Melech Hamoshiach is revealed in him“[masculine].

Regarding this logical question, it will help to take a look at the Ohr Hachama, the commentary of R’ Avraham Azulai, z”l, on the Zohar:

[…]so to it will be with the Moshiach after he merits to that neshoma and recognizes himself that he is Moshiach, as it states [in the Zohar] Moshiach will be revealed but he still won’t be recognized by the rest of the people[…] (Shemos 7b, quoting R’ Chayim Vital, z”l)

This describes how the initial “revelation” of Moshiach is when the neshoma of Moshiach is revealed to the individual who has been chosen by Hashem to be Moshiach. At the time that this individual receives the neshoma of Moshiach it is a private revelation, “but the rest of humanity will not recognize him”; only later will he be revealed to the people as Moshiach—what we refer to as the “coming of Moshiach”.

We could say that the Yalkut Shimoni’s use of the word “בו” transforms the literal meaning to “the year that Melech Hamoshiach is revealed in him”, a hint to “the year that [the neshoma of] Melech Hamoshiach is revealed in him [the one who will be Moshiach].”  In 5751 there was a revelation of the “yechida of the yechida”  (the lofty soul of Moshiach) in Moshiach himself.  The delay is the recognition of this by the world.

The reason for this “delay” is found in the Ohr Hachama on Shemos 9a, where it state:

“Moshiach can’t redeem Israel from below…only from Above…there needs to be an awakening from below in order to awaken the rachamim from Above, even if the physical Moshiach wants to redeem [them], only according to what Yisroel arouses Above can the Moshiach act….”

This describes a situation where there already exists the individual to whom has been bestowed the lofty neshoma of Moshiach (“the physical Moshiach”).  He himself knows that he is Moshiach, but the revelation is not yet recognized by all of Israel.  We can see this in the language of the Yalkut Shimoni itself: the Midrash begins with “the year in which the King Moshiach is revealed”, and the Midrash concludes with “the hour in which the King Moshiach comes”.

The year in which Moshiach is revealed refers to the initial revelation, when there is still a need to explain the events of the world to the Jewish people, “don’t be afraid”, etc. The hour when Moshiach comes refers to that transcendent moment when the Jews themselves are able to actually see and recognize the light of Moshiach (as stated in the continuation of the Yalkut Shimoni, brought in different sichos).

In other words, two distinct stages: the revelation of Moshiach which precedes and is distinct from the coming of Moshiach.  The Rebbe is informing us that the first stage has been fulfilled, and we have transitioned to the second stage, which is dependent wholly on the efforts of Yisroel, as the Ohr Hachama writes: “only according to what Yisroel arouses Above can the Moshiach act….”

Therefore, it is understood the words of the Rebbe (28 Nissan): “I have done all that I can do, now I give it over to you to do all you can do to bring Moshiach in actuality”.  If we truthfully cry out “Ad Mosai” (“until when”), we are really asking the question on ourselves: because while Moshiach is the one who brings the Geuloh, Moshiach’s ability to act comes from us.

See 5751–The Year Moshiach Is Revealed for more insights into the relevance of this Yalkut Shimoni to the Gulf War and the present world situation.