I usually frown upon these kind of threads; Statements such as "Seres/Thys is superior" or "Han-dynasty could easily crush Daxia", is originated from Chinese annals denoting both Parthian and Bactrian arms as "weak" using the Chinese military axioms as point of reference; Zhang Qian mentions a plethora of countries in his voyages,amongst these Dayuan (Ferghana), Kangju (Soghdia), Yancai/Alanliao (Alans), Daxia (Bactria), Anxi (Parthia), Lixuan (Hyrcania) and Tiaozhi (Mesopotamia), but the manner of application would have them separated as distinct countries, which is a very tricky assessment: Perhaps Zhang Qian intended to emphasize the feudal societal structure of the Parthian empire, but surprisingly he foregoes the mention of the Posse (Persians) and a number of other client states, which on the other hand implies that Zhang Qian misunderstood the nature of the Parthian empire as a de facto confederacy.

Because Parthian arms relied on the provisions of the nobility, and especially amongst the High Clans, which after the first Parthian civil war, almost gladly jumped at each others' throats, it may have been "weak" in that manner, that it was loosely tied to the allegiance of royalty. Otherwise, the Parthian empire, as a whole fielded highly trained light horse, and archers, and with a distinct champion culture, they had re-founded Iranian chivalry; This certainly intrigued the Chinese, especially during the Age of Fragmentation, Sui dynasty and at the flower of Chinese chivalry, the Tang dynasty, where the formation of extra-heavy cavalry became more common due to previous Iranian but also upcoming Turkic military influences (In which the Tarkhan champion culture draws heavily upon Partho-Sassanian chivalry, especially during Gök/Heavenly Turk migrations); Tang dynasty cavalry would take form into a very unique equestrian tradition that blossomed during the Song era.

So therefore, I'd take Zhang Qian's report with a pillar of salt. The Iranian empires under the Arsacid-Sassanian dynasties dealt with the Indo-Scythian and Yuezhi (Gondopharid expeditions), and the Sassanians under Shâpûr I destroyed the Kushans (Imposing in their stead the Kûshânshâh rule), and later the Xîyûn (Red huns/Chionites), and later the Hûnâ (White Huns/Hephtalites) under the patronage of Chosroës I "The Just", and twice gave the Gök-Turks the spanking of their lives under the campaigns of Varahrân IV Chûbîn (Lit. The Wooden Stick) and the Perso-Armenian general Smbat of the Bagratids. All the while they were flanked by marauding Arabian nomads to the south, the Romans to the west and with Scytho-Sarmatians and later Hunnic tribes to the north. If Chinese sources ascribes "weakness" on such grounds, only to be blindly accepted then quite frankly China-fanboys have the uncanny ability of taking a walk on my patience. That Mithradates I The Philhellene managed with sheer ability to tackle both the Seleucids and the Graeco-Bactrians simultaneously, with success if I may add that, is nothing to scoff at; Equally the unfortunate Eucratides I managed to stretch his empire as far west as Tapuria (Hyrcania).

That the Chinese managed to drive out the Xiongnu or the Yuezhi is not absolute proof of superior Chinese military arts; In fact, China never permanently got rid of the problem with marauding nomads, something which has been attested by the "annoying immortality" of the Partho-Sassanid eastern rival. Darius I The Great of the Achaemenids too tried to drive out the Scythians: He was successful against the Tîgrakhauda, and the Haomâvargâ, and took success for granted against the Parâdrayâ in the west. History proves to us that chasing nomads in the pursuit of getting rid of the threat, is a lesson of futility. China is far from the exception with the various periods of heavily Turkic-influenced rule.

On the issue of Sino-Greek relations, it is also inherently relevant to Sino-Iranian relations; As Keravnos mentioned, the Greek connections with the Chinese, from two ends, the Ferghana, near the Tarim, and the Champa-ports in today's Vietnam, and subsequent Kamboja migrations eastwards from India (Posited that they finally ended up in what today is called Cambodia) and mainly pertained to mercantile enterprise, something that the Greeks had inherited from the previous Achaemenid infrastructure; Upon the Parthian succession in Iran proper, these commercial ties would be not only be tightened by the formal declaring of the Silk Road (It had existed previously, so this was only a contract or a treaty of trade), as a pact between Mithradates II The Great and Wu Di of the Han. Later however the sea-trade would drastically improve once the Parthians invested in improving the ports around the Persian Gulf; The Aspasine clients in Charax would go on to make Charax one of the greatest port cities East of the Mediterranean. This later forced the Parthians to build the foundations for Sirap, another port city in the southern shores of Persis proper in order to increase the "trade bandwidth", and these were maintained by the Vâzarangîg clients of Persis proper.

From here we observe Arsacid currency as far as Vietnam and Zanzibar in Africa, where the former had taken the traditional routes to the ports in India (Barbarikon - Barygaza) and circumnavigate the area as far as the mouth of the Ganges; The latter had made use of Arabia and taken a route along the horn of Africa. An interesting tidbit is that while the Parthian colonization of certain places in coastal Africa appears unique, the route was known since the days of Sataspes when he was ordered by Xerxes to circumnavigate Libya (A task which he ultimately failed to achieve; By the simple fact that there was much, much more to Africa than what initially met the eye ). In fact, later Greek and Iranian expeditions past the Alexandrian age owes heavily to the expeditions of Scylax of Caryanda and Sataspes, employed by Darius I The Great and Xerxes I. While the expeditions to the Champa in Vietnam appears a bedazzling feat, it was in fact quite common. This was the so-called maritime Spice Route, which complemented the land-route of the Silk Road, and the land-based Frankincense Route and its later maritime incarnation in Arabia.