what I meant by "peculiar defensive requirements of the Qing" -
What I was referring to was the Qing's (fairly well-founded) perception that their security interests were most dependent on the Mongolian situation and the related Mongolian-Tibetan cultural nexus.
As Peter Perdue's book tells it, the Qings (like the Ming before them) saw proper handling of the Mongols critical, and they were determined to move past old strategies of bribery and extermination campaigns which had not worked. The Qings projected power and diplomacy into Mongol areas and basically decided that "tame" Mongols, given an honored place in the "imperial registers" were OK, but powerful Mongols under "wild" or non-submissive conditions were unacceptable.
The Qings succeeded fairly early in coopting and subjugating the eastern Khalka Mongols, but the process of subduing the western Dzhungar Mongols spanned three generations and three Emperors.
Subjugating the Dzhunghar lands required seizing not just what is far western Mongolia today, but also the whole northern part of Xinjiang (which was Mongol and Lama Buddhist, not Turkic and Muslim) all the way into northeast Kazakhstan.
The "taming of the Mongols" also required a firm Chinese influence over the Tibetans, who, through the connections of Lama Buddhism, had the ability to provide religious sanction to the leaders of all Mongols, who were also Lama Buddhist. Tibet and Mongolia engaged in the ritual tea trade, Mongol pilgrimage to Tibet, and Tibet supplied many Lamas who ministered in Mongolia.
Territorially speaking the territory of Kokonor (Mongolian name probably) a majority Tibetan speaking area (it's Tibetan name is Amdo, and it was the birthplace of the Dalai Lama) was a major focal point of this cultural exchange. Today it is the Chinese province of Qinghai.
http://en.wikipedia.org/wiki/Qinghai
So, because of ethno-religious-cultural factors, and not explicitly geographic ones, the Qings by the time of Qianlong found it made sense to treat Mongolia, Tibet and the territories of Xinjiang, Qinghai and Gansu in between them as pieces of a matched set.
The Turkic "begs" of southern Xinjiang were probably just conquered because they formed the geographical seam between Tibet and northern Xinjiang and/or they had paid tribute to the Dzhungars.
The last anti-Dzhungar campaigns were carried out in a genocidal fashion to destroy the last resisting Dzhungar tribes, which could have been a major factor in the displacement of Lamaist Mongols with Muslim Uighurs in much of Xinjiang.
In any case, the Qing did not feel compelled to attack peoples further to the north (like the Russians, who wisely declined Dzhungar Mongol overtures) or to the west like the Kazaks, Kirghiz or Uzbeks of Bukhara, because they had no relationship of consequence to the Mongol question.
Qianlong was mighty proud to have found a final solution to the Mongol problem which had bedeviled China for over 500 years, and seeing those campaigns through to victory was glory enough for him. Qianlong also did invasions or interventions in Vietnam, Burma and Nepal, but probably was able to cut his losses with greater equanimity there than he could have in what is now the western PRC, because those peoples had no connection to the Mongol question. Likewise, because the Japanese adhered to tributary form and had no connection to the Mongol question, the Qing did not feel any need to really investigate or punish the Japanese for their role in Qing tributary Liu Chiu (Ryukyu, ie Okinawa) kingdom.
So, basically, in the Qing era, the inland frontier was key, and the maritime frontier could not compete for attention. korea and Japan were quiescent, and European were even more quiescent in Chinese waters in the 1700s than they had been in the 1600s. China had held its own against the Dutch and the Spanish, who were in any case coming to terms with their limits by around 1700. The British and French spent 1700-1815 fighting each other but in Asia they were preoccupied with India, and their own fights had to be resolved before the cost-benefit of pushing into China seemed worth it.