Al-Isra: 7
"If you do good, you do good for yourselves..."
Meaning: Both the good and the evil [you do] are restricted to your own selves; the benefit and the harm do not extend to anyone else.
It is narrated from Ali (may Allah be pleased with him): "I have neither done good to anyone nor done evil to anyone," and he recited this verse.
"...And when the promise of the second [time] comes..."
We have sent them [to you]—the object is omitted because its mention earlier indicates it.
"...to sadden your faces..."
Meaning: To make the traces of grief and distress appear upon them, as in His saying: "The faces of those who disbelieved will be distressed" (Al-Mulk: 27).
It is also recited as li-yasu'a (to sadden), where the pronoun refers to Allah, or to the promise, or to the sending. It is also recited as wa-li-nasu'a (and that We may sadden) with the nun. In the recitation of Ali, it is li-nasu'anna and li-yasu'anna (with the light nun).
The lam in "to enter the mosque" is, according to this, connected to an omitted phrase, which is: "And We sent them to enter [the mosque] and to sadden [your faces]."
"...and to destroy what they had overcome..."
The object of li-yutabbiru (to destroy) is ma (what), meaning: to destroy everything they had overcome and seized. Or, it may mean: the duration of their dominance.
"It is expected that your Lord will have mercy upon you, but if you return, We will return. And We have made Hell a prison for the disbelievers."