The following is a freely-translated excerpt from a letter by the Rebbe written in the opening days of the year 5732 (September, 1971):
As we know and see, there are situations in which Jews, regardless of their best intentions, regardless, even, of their self-sacrifice, are truly unable to fulfill the will of G-d due to circumstances beyond their control. As the parable goes, self-sacrifice can enable one to jump off a roof down to the ground, but it cannot enable one to jump from the ground up onto the roof...
But G-d is the master of the entire world. Since G-d Himself has commanded and spelled out how every Jew should behave in his daily life, how is it possible that there should be a situation in which a Jew is unable to carry out the will of G-d, in every detail?
There are two components to a mitzvah: the deed, and the kavanah and feeling that accompany it. It is true that “the deed is the primary thing,” but the kavanah and feeling are also of great importance.
When it happens that there is a situation in which it is impossible for a Jew to actually carry out the will of G-d despite his self-sacrifice, this stimulates in him a deep spiritual pain that pervades him to the very core of his soul, bringing him to a deeper connection with G-d, and with Torah, mitzvot and his Jewishness, the likes of which he could never have attained without this painful experience.
The fact that he did not actually do the mitzvah has no adverse effect on his relationship with G-d, since he was prevented from doing it by forces beyond his control. On the other hand, the feeling element of the mitzvah achieves a height otherwise unobtainable, and the experience imparts to his spiritual life a depth and perfection that only this situation can generate.