I’ll try to answer your questions as best I can in several parts.

1. The collapse of the Safavid state occurred due to a combination of long-term economic decline which got fused with medium-term political and ideological developments. The incompetence of Shah Hossein, rise of ulemas, faction fighting among the ruling class, which involved Georgians versus Persian nobility, etc. all played a part. The Safavid system was structurally designed tor an autocrat like Shah Abbas I. The state system centered in the Shah, and needed a firm, relentless, inquisitorial personality at the centre to keep it functioning properly. In the latter part of the 17th century, with no serious external threats or internal challenges to his authority, the ruling Shah let things slip. He gave less and less attention to government, leaving responsibility to the chief vizier and others. No doubt there were efficient, energetic men among these officials, but when they saw their efforts disregarded by the Shah, and others profiting for themselves and ignoring the interests of the state, they became discouraged. Gradually a miasma of inefficiency and neglect descended upon the system. The economic decline of Persia also took place in this time. While its effects are probably overdrawn, and while the Persian economy didn’t necessarily decline in revenue, GDP, standards of living, or structure, it failed to keep its position in the world economy. The most important export commodity for the Safavid state, silk, began to lose trade value as it was replaced by Bengali silk. Further, cotton from South Asia flooded Iranian markets. It was the continued production and growth of a subcontinent with over 150,000,000 people, which contributed to the decline of Persia in the world economy. Another cause was the tribal resurgence that took place in Persia in this period. Even Nader Shah, the savior of Iran, was a product of this resurgence. The principle cause of decline, was of course, the end of a powerful and competent autocrat at the head. When Shah Sultan Hossein’s father Shah Soleiman lay dying in 1694, he left it undecided which of his sons should succeed him. Soleiman told his courtiers and officials that one of his sons, Abbas Mirza be raised to the throne. Once Soleiman was dead the eunuch officials who guarded and supervised the harem decided for Sultan Hossein, seeing that he would be easier to control. His great-aunt, Maryam Begum, the dominant personality in the harem, dominated him and was able to control power in the state. The Pashtun rebellion was ignited because of the brutal policies of Sultan Hossein, which cracked down on the traditional lifestyle of those living south of the Hindu Kush. This erased decades and decades of semi-tolerance against the Pashtuns and Sunni Muslims, and as a result the competent and wise Mirwais Hotak, head of the Ghilzai tribe, led a rebellion. The Iranian army was not weak when it remained somewhat stable. Abdullah Khan, the Georgian commander appointed to quell Pashtunistan, defeated the Ghilzais/Hotaks and captured Mirwais, bringing him to Isfahan. But Mirwais was never executed, as due to his wits and diplomatic prowess, he was able to get out of Isfahan by appeasing the Safavid elite. Mirwais also saw for himself what was actually happening at the court in Isfahan, and the weakness of the Sultan and his state. Though the Hotaks struggled for the rest of Mirwais’ reign, the Hotaks only grew in power. The Abdali tribe (the traditional rival of the Ghilzais), joined the Hotaki struggle, and the Persian authority collapsed. The Afghans ascended, won a decisive victory at Gulnabad, and then razed Isfahan. The Iranians failed to win at Gulnabad because of the impatience of their commanders (Rostam Khan and Sayyid Abdullah), along with the high ground and favorable positions the Afghans, though outnumbered (42,000 vs 11-20,000), maintained. Once Isfahan fell, the Ottomans and Russians came from all sides. Both governments had sent missions to Isfahan, and discovered the instability of the state. Both saw this as an opportunity, especially the ambitious Peter the Great. Both states expanded into Iran. The Russians through the Caspian Sea and into the Caucasus, and the Ottomans in Iraq as well as the Caucasus. The Persian campaign was one of Peter the Great’s ‘Great’ campaigns, and it resulted in Tahmasp (son of Sultan Hossein) accepting the cession of Darband, Gilan, Mazanderan and Astarabad to the Russians.

2. Vakhtang VI, the Georgian King of Kartli, had invited Peter the Great into the Caucasus for his campaign. After Peter won his victories, he left Vakhtang and did not keep a stable presence in the Caucasus. Vakhtang VI eventually went to Russia when Peter invited him, just before he (Peter) died. Vakhtang’s kingdom was now vulnerable to attacks from the Iranians and Turks. The rise of Nader Shah was the principle cause of all of this. Nader was a mere freebooter aiding the Shah when the Safavids fell, and rose to be the brilliant commander and savior of the Safavid dynasty. Nader Shah was one of the most gifted generals in world history, and arguably the greatest early-modern general until Napoleon and Arthur Wellesley. His brilliant tactics, war strategy, courage and hardiness made him nearly invincible in the battlefield. He improvised tactics based on the conditions of the field, and made excellent decisions to prepare. It was Nader’s immediate and brilliant victories from 1730-32 against the Turks and the decade before (against the Afghans, and the victories which saved the Iranian state) which persuaded the Russians to come to a treaty. The Russian forces were lazy in the region after the death of Peter, and the Russian government knew that fighting Nader was not the best option. Rather, they decided to compromise, and unite against a common enemy in the Ottomans. The Russians who had cooperated with the Ottomans to ‘partition’ Persia just a few years before, were now ready to come to terms with Nader. Thus, the Treaty of Resht was signed in 1732. The treaty allowed Vakhtang to return to his kingdom, and recognized a common border between the Russians and Persians, returning some of the lands captured on the Caspian coast. The treaty created peace between the Russians and Persians, and allowed Nader to continue his war against the Ottomans.

The decisive battle which brought an end to the Ottoman-Persian War (1730-36) was the brilliant Battle of Yeghevard. Arguably, this masterpiece in the field was Nader’s greatest battle (excluding maybe Karnal). Nader strategically camped near Yeghevard, and positioned his flanks perfectly. He was heavily outnumbered, with the Ottomans having over 80,000 soldiers while he had a mere 15,000 soldiers at most. Nader maneuvered his jazayerchi (musketeers, many armed with flintlocks) on one wing of Ottoman artillery, and it caused dismay amongst the Ottoman artillery. Nader had preplanned a maneuver for his other wing to cooperate with one side, and to destroy the Ottoman artillery. With both of his flanks advanced, the Ottoman artillery collapsed. Nader then thinned the lines of the Ottoman centre with his mobile artillery and outnumbered though effective ranks of infantry. He then deployed his cavalry immediately as the Ottomans began fleeing and scattering. What followed was an absolute slaughter on the battlefield with Turkish Commander Koprulu Pasha being killed, and much of his force being destroyed. The conventional estimate is that 40-50,000 were killed, but this may be exaggerated.

According to The Sword of Persia: Nader Shah, from Tribal Warrior to Conquering Tyrant, Michael Axworthy

Nader himself reported that the Persians “cut all the janissaries to pieces, so that not one soul of them escaped” as well as “a great number” of the Ottoman cavalry, and that, “by the Grace of the Most High, Almost their whole Army is kill’d”.

Nader’s victory at Yerevan ended the war, and the Russians were willing to negotiate another treaty. The Treaty of Ganja created a defensive alliance between the Russians and Iranians against the Turks, as the Russians now prioritized the military mastermind that was Nader Shah, over the Ottomans. The Treaty also defined the borders in the Caucasus, and ceded the Southern portion to Nader. By 1736, Nader Shah had virtually reconquered all of his lost territories from the Russians, Ottomans, and Afghans. He would be coronated as Shahanshah of Iran in the same year.

To conclude, individuals played the biggest part in the destruction and resurgence of Iran. Brilliant leaders like Mirwais Hotak and Peter the Great carved out Iranian territory, as incompetent leaders like Sultan Hossein allowed their empire to fall. The rise of Iran was once again, because of the heavenly-born general that was Nader Shah. The death of Peter the Great caused the Russians to give up their aspirations in the Caucasus (temporarily, of course) and incompetent generals and leaders like Koprulu Pasha and the tribal divisions within Afghanistan allowed for Iran to make a resurgence as well. In fact, the incompetence and weakness of Mughal Emperor Muhammad Shah also would play a part in Nader Shah’s great Sack of Delhi. Ultimately, while military and economic advantages existed, it was mostly individuals which allowed for the conditions of Persia to transform so rapidly in a decade.