On the 9th of Av the stronghold city of Betar was captured and destroyed by the Romans (~135 CE) (Mishnah, Ta'anit 4:6)

Forty baskets of Tefillin (phylacteries) were found on the heads of those who were killed in Betar. (Gemara Gittin 58a)

Bar-Kochba, based in Betar, led an army of G-d-fearing Jews to who rebeled against the mighty Roman empire that ruled over Israel. His forces were eventually overcome by the Romans and slaughtered while still wearing their Tefillin. What is the significance of the fact the Bar-Kochba's soldier's were cut down in their Tefillin? Is the Gemara simply reporting the piety of Bar-Kochba's troops?

II The Gemara describes a unique quality of Tefillin:

Why did Elisha [a righteous man who wore his Tefillin in defiance of the Roman decree against wearing them] refer to his Tefillin as dove's wings? ...Just as a dove's wings afford it protection [a dove wards off enemies with its wings -Rashi], so too, the Mitzvot [that is, particularly the Mitzvah of Tefillin] protect the Jewish people. (Gemara Shabbat 130a)

If one speaks after donning Tefillin on the arm, but before completing the Mitzvah by donning a second Tefillin on the forehead, it is considered a sin. One has sinned in such a manner may not join ranks of the Jewish army [as he will not be promised divine protection]. (Gemara Sotah 44b)

Not one of the warriors [who fought against Midyan, in Bamidbar 31] wore their forehead-Tefillin before their arm-Tefillin. Had they done so, Moshe would not have praised them and they would not have all returned home safely. (Midrash Shir HaShirim Rabba to verse 4:4)

It is through keeping the Mitzvah of Tefillin [on the arm and the forehead] properly that Hashem grants the Jewish armies the blessing of Moshe Rabbenu, "He shall smite the enemy's arms and foreheads" (Devarim 33;20 -- Rashi: They would sever the head and arm of the enemy with one blow). (Rosh, Hilchot Tefillin sec. 15; see also Kol Eliyahu, #132)

III What is it about Tefillin that affords its wearer protection in times of war? And why is it so important not to don them in reverse order and not to interrupt the process of donning Tefillin by speaking? In Berachot we are told:

Hashem too wears Tefillin.... What is written on the parchment enclosed in the Creator's Tefillin? "Who is like Your nation, Israel, a unique nation on earth! (I Divrei Hayamim 17:21)".... Hashem says, "You, Israel, have proclaimed Me unique, as it is written, 'Hear O Israel, Hashem is our Lord, Hashem is One (Devarim 6:4),' I too shall proclaim you unique, as it is written, 'Who is like Your nation Israel, a unique nation on earth!' " (Berachot 6a)

Although the "Tefillin of Hashem" is obviously a metaphor, the Gemara's theme is clear. Tefillin represent the reciprocal relationship between the Jews and our Creator. The passages enclosed in our Tefillin proclaim, "Hear O Israel...", while those enclosed in His proclaim, "Who is like Your nation Israel...."

A survey of the four passages written on the parchment enclosed in our Tefillin reveals the same theme. The first two, Shema (Devarim 6:4 ) and Vehaya Im Shamo'a (Devarim 11:13), announce our commitment to Hashem and His Mitzvot (see Mishnah, Berachot 13a), while the second pair of passages, Kadesh and Vehaya Ki Yeviacha (Shemot 13:1), remind us of the wonders that Hashem did for us when He freed us from the Egyptian bondage. Through our commitment to Hashem, we merit Hashem's commitment to our protection. (According to the Gemara in Sanhedrin 4b, the Torah itself classes the portions enclosed in the Tefillin into two groups, referring to Tefillin as "the 2 + 2.")

This dichotomy is apparent in the two parts that make up a set of Tefillin -- arm-Tefillin and forehead-Tefillin. The arm-Tefillin is worn parallel to the heart (Menachot 37b), representing our love for Hashem. It is worn on the weaker arm (ibid. 37a), to remind us that before Hashem, we are powerless. The forehead Tefillin, on the other hand, demonstrate Hashem's relationship to the Jewish People:

Where do we find that Tefillin are the strength of Israel? The verse states, "All the nations of the land will see that Hashem's name is upon you and they will fear you (Devarim 28:10)." ... When? When they see the Tefillin that is on our heads. (Gemara Chullin 89a)

The forehead-Tefillin are also termed "our glory" (Gemara Berachot 11a) -- they are the crown with which our Creator has crowned us.

This is the secret of Tefillin. Through our faith in Hashem, we merit His protection from our enemies and we are crowned as the nation that is above all others!

We can now understand why the arm-Tefillin must be donned first. Only after we demonstrate our commitment to Him (= the arm-Tefillin), does Hashem respond by crowning us with His glory (= the forehead-Tefillin). One who reverses the order demonstrates that the chosenness of the Jewish People is not a direct result of their commitment to Hashem, and such a person is doomed to defeat. Similarly, if one dons his arm-Tefillin and then interrupts by speaking before donning his forehead-Tefillin, he does not recognize the his "crown of glory" is a direct result of the deference he pays to Hashem. He, too, shall fall.

IV Why was the city of Betar defeated?

When Bar-Kochba would go to war [with his 400,000 mighty warriors] he would declare, "Master of the universe, I don't need your help -- just don't hinder me!" (Yerushalmi Gittin 4:5)

The downfall of Bar-Kochba's army stemmed from the fact that they did not put their faith in the Creator, but in their own physical prowess. Consequently, their Tefillin were unable to protect them and "forty basketfulls of Tefillin" were found on their dead bodies. The Gemara in Gittin with which we began continues to describe exactly how many Tefillin were found on the dead warriors of Betar.

Forty baskets of Tefillin (phylacteries) were found on the heads of those who were killed in Betar. Rebbi Yannai son of Rebbi Yishmael said: Three containers, each containing forty basketfulls, were found.... The two opinions do not disagree: One is discussing arm-Tefillin while the other is dealing with forehead-Tefillin. (Gemara Gittin 58a)

The Vilna Gaon explains the conclusion of the Gemara as follows: The invading legions caught the Jews of Betar just as they were praying. Some Jews were still donning their Tefillin, others had already donned them, while yet others had already begun to remove them. Since the arm-Tefillin is donned first and removed last, the Jews in all three stages of Tefillin-dress wore arm-Tefillin, while only those in the middle stage (wearing both) had on forehead-Tefillin. This is why three times as many arm-Tefillin were found! (Kol Eliyahu, #222)

According to the Gaon, it may be said that the sudden murder of the Jews of Betar while praying reveals the sin that caused their deaths. Because they "interrupted" the Mitzvah of donning Tefillin -- that is, they did not attribute their physical prowess to Hashem -- a good number of them were slaughtered while they were wearing the arm-Tefillin without the forehead-Tefillin!