The fire of Torah that opposition only strengthens

כתיב ויסעו מרפידים ויבאו מדבר סיני.

It is written, "And they journeyed from Refidim and came to the Wilderness of Sinai" (Shemos 19:2).

The Torah connects the departure from Refidim — where Amalek attacked — directly to the arrival at Har Sinai for Mattan Torah, and the Sefas Emes explains why these two are linked.

דכתיב מים רבים לא יוכלו לכבות כו' האהבה דתורה בחינת אש כמ"ש כה דברי כאש.

For it is written, "Many waters cannot extinguish [the love]" (Shir HaShirim 8:7) — the love of the Torah is the aspect of fire, as it says, "Are not My words like fire?" (Yirmiyahu 23:29).

The love bound up in Torah is like a fire, and "many waters" — the forces that try to cool it — cannot put it out, as the navi compares Hashem's words to fire.

ודרך האש כשבא עליו מים ואינו נכבה.

And the way of fire is that when water comes upon it and it is not extinguished —

It is the nature of a strong fire that when water is poured on it but fails to quench it, something further happens.

מתגדל עוד ביותר כח והתלהבות.

it grows even greater in strength and burning passion.

A fire that withstands water blazes up even more intensely. So too the love of Torah, when challenged by opposition, only grows stronger and more fervent.

וכ"כ ולאום מלאום יאמץ.

And so it is written, "and one nation shall prevail over the other" (Bereishis 25:23).

The struggle between the two nations — Yaakov and Eisav — is part of this dynamic: the opposition of Eisav/Amalek serves to strengthen Bnei Yisrael, like water that intensifies the fire.

ומזה המלחמה של עמלק באו לקבלת התורה:

And from this war with Amalek they came to the receiving of the Torah.

The battle against Amalek at Refidim was itself the preparation for kabbalas haTorah: the attempt to cool Bnei Yisrael's fire only inflamed their love for Hashem, propelling them straight to Sinai.

Summary: The Torah links the journey from Refidim (the war with Amalek) directly to Har Sinai because the love of Torah is a fire that "many waters cannot extinguish." Like a fire that only blazes hotter when water is thrown on it, Bnei Yisrael's love for Hashem was intensified, not cooled, by Amalek's attack — so the very war with Amalek became the preparation that brought them to receive the Torah.