The yom tov of Pesach, as we know, corresponds to two events, the first day to the actual exodus from Egypt and the last day to קריעת ים סוף. Let us try to gain some insight into the special character of שביעי של פסח by looking at the response of Klal Yisrael to the miracle – the שירת הים.
אז ישיר begins: אשירה לה' כי גאה גאה סוס ורוכבו רמה בים.
This פסוק must capture in some way the essential theme of the שירה as it constitutes the entire שירה that מרים later says. To understand the שירה we must first understand this pasuk:
1) What does סוס ורכבו רמה בים mean? Rashi comments: שניהם קשורים זה בזה - horse and rider went down together. Why is this so significant?
2) כי גאה גאה- Onkelos translates: אתגאי על גיותניא which perhaps can be rendered, “He triumphs over those who think themselves triumphant.” The pasuk seems to connect this idea with סוס ורכבו רמה בים - what is the connection?
3) Chazal say: ראתה שפחה על הים מה שלא ראה יחזקאל בן בוזי. They connect the experience of קי"ס with the vision of יחזקאל. Bnei Yisroel experienced tremendous גילויים at יציא"מ, even more at הר סני – but the particular גילוי of קי"ס is somehow associated with the vision of יחזקאל. Why is this so?
יחזקאל begins his נבואה with מראות אלקים ,מלאכים and חיות and riding above them, ,כביכול the רבש"ע Himself. חז"ל have a special term for that vision — מרכבה — and they considered it the most hidden, the most esoteric part of Torah. Obviously, the study of the מרכבה is not for us. The Mishna in Chagiga tells us ולא במרכבה ביחיד — even Chazal were reluctant to share this branch of learning with their students. Nevertheless, perhaps we can at least get a sense of what the concept of מרכבה signifies.

A מרכבה is a chariot. What drives a chariot? Horse-power. The horses careening around feel their own strength, they exhilarate in their own power. To them it seems that they are driving the chariot. But, of course, it is really a מרכבה — something driven, something controlled. It is the רוכב על המרכבה who directs the מרכבה as he chooses.
This is an apt metaphor for a truth that the Torah teaches again and again. It is a central motif inתנ"ך - the idea that for all of man’s efforts, ultimately it is the רבש"ע who is in control, who is, so to speak, in the driver’s seat. There is a paradox here. Not only is Hashem’s will manifested despite man’s actions, but precisely through man’s actions. And nowhere is this more evident than when the very efforts of a רשע to perpetrate evil rebound against him. This itself is a theme that repeats throughout תנ"ך. To take two famous examples: פרעה decrees that the first born should be thrown into the river – to thwart the גאולה which his astrologers predict – and as a result Moshe is found by פרעה‘s own daughter and raised in his own palace where Moshe is groomed for leadership. Or, המן has a gallows erected in anticipation of having Mordechai killed, only to facilitate his own demise on the very same gallows.
That is the idea of מרכבה. The horsepower of history, the motive power, is the actions of men. Yet at the end of the day it is a מרכבה something driven, under control. (Of course, this has to do with the mystery of ידיעה and בחירה - the seemingly contradictory postulates that man is a free agent and that הקב"ה shapes his affairs to His own ends. The Rambam writes that it is beyond human comprehension how these ideas can be reconciled. That is part of the mystery of the מרכבה.)
Let us follow this line of thought further. סוס ורכבו רמה בים. The מצריים are destroyed even while still riding their horses and chariots. A man on horseback is a symbol of power. Statues often depict military leaders as riding on horseback. And Egypt especially is associated with that kind of horsepower – לא ירבה לו סוסים ולא ישיב את העם מצרימה למען הרבות סוס. The chariot was the particular Egyptian war machine. The meaning, therefore, ofסוס ורכבו רמה בים is that the Egyptians were destroyed precisely at that moment when they felt themselves to be most powerful, most puissant, most fearsome. At that moment when they thought themselves to be hurtling on the enemy – they were being hurtled towards their own destruction. The very horse that was their most fearsome weapon – dragged them into the waves.
The Egyptian army thought that they were the riders, they were in control. And suddenly it emerged that they were not the drivers, but the driven; not the רוכב, but the מרכבה. And that is the meaning of the pasuk – אשירה לה' כי גאה גאה – He is מתגאה על גאים, He overpowers the powerful – at the height of their power. He doesn’t need to gradually bring them down; at the very height of their power, he drives them to their own destruction. As He did with the Egyptians – סוס ורכבו רמה בים.
What we witnessed, therefore, at ים סוף was a revelation of the מרכבה, of הקב"ה as the driving hand behind history, even behind the actions of the Egyptians, as they drove themselves into the sea. And therefore ראתה שפחה על הים מה שלא ראה יחזקאל בן בוזי — because יחזקאל saw the מרכבה in a vision, but at ים סוף we saw the מרכבה revealed in life itself.
The lesson for us is very real. We too see מרכבות פרעה וחילו, the weapons of our enemies brandished with blood curdling threats – threats that too often are carried out. It is easy to be disheartened, even fearful. It is, therefore, so important to remember, and to carry in our hearts the words of the שירת הים. אשירה לה' כי גאה גאה - I will sing to Hashem because he triumphs over His enemies – even in the very moment when they seem most triumphant; סוס ורכבו רמה בים - from the height of their power they are cast into the sea. כן יאבדו כל אויבך ה' - בב“א.