Hijab is one of the Islamic duties which existed in other religions also. It has been stated in the Holy Quran and in numerous traditions that a woman should cover all parts of her body except her face and hands up to the wrists. Therefore, the enactment or legislation of modest covering has nothing to do with the Islamic and non-Islamic governments or systems. When it comes to a woman's private life, hijab is an individual and private matter which is all up to her; no one has the right to compel her to observe hijab in her privacy. But when hija turns into a social matter and relates to others' rights, the government or the ruler can make the observance of hijab mandatory and can punish the offender in a bid to prevent vice and preserve morality and spirituality in society. That is because immodesty is considered to be a divine prohibition. According to Islamic principle, violation of a religious prohibition is punishable.
Therefore, the duty of all the Islamic governments and administrative systems is to execute and enforce all Islamic commandments. Unfortunately, most of the Islamic states are negligent for failing to take action in this regard.
Hijab is one of the Islamic duties which also existed in other religions long before Islam. The Islamic modest dress is so important that the Quran and many traditions have made mention of its rules and limits. In Sura al-Noor God says:
"Say to the believing women that they cast down their glance and guard their private parts and reveal not their adornment except such as is outward and let them cast their veils (khumar) over their bosoms and reveal not their adornment except to their husbands, their fathers, or their husbands' fathers or their sons or their husbands' sons, or their brothers or their brothers' sons, or their … nor let them stamp their feet, so that their hidden ornament may be known."
It has been explicitly stated in the above verse that covering all the adornments, head and neck is necessary for a woman in her associations with men 'whom she is not related to according to the Divine Law' (na-mahram).
Many believe that the face and the hands from the wrist down are exception from the rule, and there are indications of this exception in this ayah; such as:
a) The exception of apparent beauty in this ayah, whether it denotes beauty itself or the place of beauty, is a clear explanation as to why it is not necessary to cover the face and hands.
b) The meaning of the commandment of this ayah is another indication for this claim in regards to throwing the ends of a head-garb or covering over the collar-area; covering the whole head and neck and chest, yet it does not mention covering the face. History has also shown us that wearing a face-veil was not a public custom in the beginning of Islam.
In several traditions, the pure Imams (Peace be upon them) , in the exegesis of this noble ayah, have explained the amount of hijab tha is necessary and obligatory. Fuzail Yasar (one of the companion of Imam Sadiq (Peace be upon him)) said, I asked Imam sadiq (Peace be upon him), “Are the forearms (from the wrist to the elbow) of a woman one of the beauties included in those which Allah has stated that a woman should not reveal to any other than her husband?” Imam Sadiq answered, “Yes and the body parts which are lower than the head-garb are considered beauties…”
Also Masadah Ibn Zararah has narrated from Hazrat Imam Sadiq that when he asked the Imam about the beauties that a woman can expose [in public], he stated; “the face and the hands.”
Of course we must pay attention to two points:
1. From an Islamic perspective, revealing of a woman’s face has no problem as long as it is without makeup, or with very minimal makeup (meaning it is not considered makeup by the common culture), and is not a cause of corruption.
2. The instances which explained that the face and hands do not need to be covered do not indicate that there is nothing wrong with a man looking at them; because there is no necessity between the two, and here we have only discussed the first issue.
Therefore, the legislation and legalization of Islamic modest dress and determination of its limits are the duty of God and the Holy Prophet, peace be upon him and his family. This duty has been fulfilled in the best manner. However, it is the duty of the Islamic governments and states to enforce the laws and injunctions especially those that deal with women's modest dress (because of its impact on the moral well-being of society and on solidification of family foundation).
When it comes to a woman's private life, hijab is an individual and private matter and it is a up to a woman herself whether she decides to wear hijab or not; no one has the right to interfere or conduct any investigation about it. But when it turns into a social matter and relates to others' rights, the government or the ruler can make the observance of hijab mandatory and punish the offender in a bid to prevent vice and preserve morality and spirituality in society. That is because immodesty is considered to be a divine prohibition. According to Islamic principle, violation of a religious prohibition is punishable. The punishment is also conducted under the supervision of the Islamic ruler. That is to say, from a jurisprudential viewpoint, the qualified Islamic ruler can subject an offender to punishment so as to prevent vice and divine prohibition from spreading.
Unfortunately, many Islamic states continue to be negligent in this regard as a consequence of which moral corruption and divorce rates have visibly increased. You must have heard the sad story of Andalusia or today's Spain and the colonial centers' greed and ravenousness to spread the West's sexual morality among Muslims. If the beautiful, attractive and flirting girls put their beauty on display and flaunt themselves in front of the pure but energetic and lustful youths, is preventing them from doing so considered to be an intrusion of their privacy? There is no doubt that interference on the part of a healthy Islamic authority as a bidder of good and forbidder of bad guarantees moral and spiritual well-being of society provided that it is devoid of any extremity or violence.
Thanks are due to Allah, the women in Iran and many other Islamic countries have been and are the real advocates of the Islamic modest dress, albeit with its various degrees. As you may have read the history, they resisted against Reza Khan and his son's policy to compel women to remove their veils. Even today, we witness beautiful scenes of Muslim women battling and defending their rights in non-Muslim and so-called democratic countries that are preventing, under lame excuses, the scarf-wearing women from entering scientific and academic centers.
 - Al-Noor, 31
Note: in the occasion of revelation of this ayah, they have said that Arabs during that time wore head-garbs and covering, and they would throw the ends over their shoulders or on their backs, in such a way that the head-garb would be set behind their ears, and they would only cover their heads and the back of their necks. However, below the throat and some of the chest would be visible. Islam corrected this, and commanded them to use the ends of their head-garbs to cover their chest and collar area instead of just behind the ears or the back of the neck. The result was that just the roundness of the face would be visible and the rest of the body, covered.
 Nemuneh Commentary: Vol.14, Pg.450,451
 Using Hadith from Usul Kafi: Vol.5, Pg.521, Section "That of a Woman Which is Permissible to See"
 Wasa'ilu Shia Vol.20, Hadith No.25429, Pg.203, Section "That of a Woman Which is Permissible to Look at Without Lust"
 Replies to Religious Questions by Imam Khomeini (May Allah have mercy on his soul): Vol.3, Pg.256, Ques.33, 34;  Catechism of Imam Khomeini: Vol.2, Pg.929, Section "Women and the Revealing of Makeup".
 Hijab by Shahid Mutahari (May Allah have mercy on his soul):pages 164 - 235
 - Turkhan, Qasim, Hawzah Net, question No.02-5988.
 - Al-'Anawin al-Fiqhiyah, vol.2, pg. 627, Rule (4), Kullu ma lam yarid fihi haddun min al-Shar'a fil ma'asi, fa-fihi al-ta'zir.
 - Turkhan, Qasim, Hawzah Website, 02-5988