S. R. Faruqi:

This verse recalls Ghalib's letter in which, describing the destruction of Delhi, he has written that from the Red Fort to the Jama Masjid the whole city has become a maidaan . In Mir's verse, ;xaraabah (meaning 'desolation') is of course full of meaning. In the second line kaahe ko too has two meanings: one is, 'when was such a ruin spread out here formerly?'; and the second is, 'after all, what crime had this city committed, that such destruction was inflicted upon it?'. On both readings, the 'city' is not only the city of Delhi, or any city at all-- rather, it can represent the whole world. The whole world looks like a ruin. If instead of 'has become a plain' there were 'has become like a plain', or 'has become ruined', or some other such direct phrase, then the power of the line would have become much less.

Mir has used this theme in a much inferior style, elsewhere in the first divan [{491,1}]:

be-yaar shahr dil kaa viiraan ho rahaa hai
dikhlaa))ii de jahaa;N tak maidaan ho rahaa hai

[without the beloved, the city of the heart is becoming desolate
as far as it would be visible, it's becoming a plain/field]

Giving to the 'city' the explicit identification of the 'heart', he has washed his hands of generality, or rather of universality. In the second line the image of the 'plain/field' bestowed on the heart the level of a desert or wilderness, but because of the unnecessary phrase 'as far as it would be visible', the power of the line has become much less.

The word ;xaraabah he has used in the same framework very superficially, also in the first divan [{107,8}]:

ab ;xaraabah hu))aa jahaan-aabaad
varnah har ik qadam pah yaa;N ghar thaa

[now it has become a ruin, Jahanabad
otherwise, at every single footstep, here, there was a house]

In the present verse, the word phailaa , having come together with ;xaraabah , has become full of meaning. When a city grows, they call this its 'expansion' [phailnaa]. Here, ruin is expanding/spreading. That is, the city's ruinedness is expanding, and the city is steadily contracting. The picture that forms is that in all four directions, for a very long distance, there's an uninhabited plain, or ruins and rubble, and in the midst of it is located a single small city, and every day it more or less keeps contracting further, and the plain and the ruins advance. He's composed an unusual verse.

[See also {1230,5}.]

FWP:

SETS
MOTIFS
NAMES
TERMS == INSHA'IYAH; THEME

What works against any direct reference to the vicissitudes of Delhi in particular is the idea of ruinedness as 'having spread in every direction'. As SRF observes, this is normally how cities grow. If it's applied to the decline or decay of a city, it doesn't suggest a sudden violent attack from an external army, such as Delhi repeatedly experienced during Mir's lifetime. On the contrary, in fact: it sounds gradual and organic. The city seems to have suffered from a blight of some kind. Thus the question in the second line: what had been going on here? 'This kind of ruinedness' is what elicits the speaker's question; it clearly wasn't any normal kind of ruinedness, but something quite unusual.

A plain is 'an open field (without buildings)'; it's a large clear area suitable for games, or battles, or strolling around (see the definition above). Somehow the city had been becoming not even a ruin, but rather a natural blank space, as though it had never existed. And the insha'iyah second line, which leaves us with a lingering, unanswerable question, is far more effective, as SRF notes, than any flat, explicit equation of city and heart.

SRF says that the process of ruination is still happening-- 'every day' the ruins are expanding, and the city is contracting. But the grammar of the verse puts the action entirely in the past. According to the first line, there's not even any city left at all-- there's just a level plain or field. The speaker is wondering about the nature of a process that's now utterly, irretrievably complete.

Just for nostalgia, here's an evocative view of the extensive ruins that once lay to the south of Delhi, from Robert Montgomery Martin, 'The Indian Empire', vol. 3, c.1860: