(The glorious battle of Alexander, King of Macedon, and Porus, King of India. Russian lubok via Wikipedia)
In early 327 BCE, half of Alexander’s army marched through the Khyber Pass and reached the shores of Indus. After subduing the hill tribes, Alexander and rest of the army joined them in 326 BCE at Ohind at the border of Takshashila — a large and prosperous city between Indus and Jhelum. Alexander’s activities, mostly invasion, produced different reactions from three people — two kings (Ambhi & Porus) and a king maker (Chanakya).
After a 30 day rest, Alexander crossed the Indus into “the country of Indians” and on the other side he was met by an army in battle formation. This was highly unexpected. The king of Takshashila, Ambhi or Oomphis, had sent word that he would not oppose Alexander and would fight on his side. When it looked as if Ambhi had reneged on his promise, Alexander ordered his army to get ready.
Ambhi rode up alone towards the Greeks and he was met by Alexander who too rode up alone. Realizing that what came from Alexander’s mouth was all Greek, interpreters were summoned. Ambhi explained that he had come to put both his army and the kingdom at Alexander’s disposal. He also gifted elephants, large sheep and 3000 bulls to Alexander prompting the Greek to ask Ambhi if he as into husbandry. A satrap — Philip of Machatas — was appointed to govern.
Enjoying Ambhi’s hospitality, Alexander sent word to the neighboring kings to meet him and pay tribute. While few did, one king stayed away: Porus, who was not going to follow Ambhi’s foot steps. When Alexander’s envoy met Porus and asked him to meet the emperor and pay tribute, Porus replied that he would definitely come to meet the emperor, but with an army. Thus in the spring of 326 BCE the two armies met on the banks of Jhelum.
We only have the Greek account of the battle and hence the exaggeration has to be discounted. 20,000 infantry and 3000 cavalry of Porus was killed. All his chariots were destroyed, his generals were killed, so were two of his sons. According to the Greek historians — Diodorus, Arrian, Plutarch — the Greek losses were not so high. But still Porus was praised: “his courage matched his body vigor”, “he exhibited great talent in battle performing deeds not only of a general but also of a valiant soldier.” This battle, Battle of the Hydaspes, was immortalized by Western painters like André Castaigne ,Charles Le Brun and artists in Russia.
Finally the two met. In the meeting Alexander asked Porus how he wished to be treated and Porus replied, “As befits a king”. This reply under adverse conditions impressed Alexander and he returned Porus back to the throne and turned him into an ally.
So was Ambhi a traitor for aligning with a foreigner? In his book India: A History John Keay mentions that though Porus surrendered only after giving Alexander a good fight, calling Ambhi  who surrendered without a fight a traitor is  harsh judgement. An argument is that there was no concept of India as a nation and if a king like Ambhi took help from Alexander to be safe against attacks by Porus, can he be blamed?
That argument would have held, if not for the efforts of Chanakya, who saw the cultural unity among the various kingdoms. As a teacher in Takshashila, he saw students — brahmin youth, princes, sons of rich merchants — come from far away places along the uttarapatha to learn the Vedas, arts (archery, hunting, elephant lore, political economy) law, medicine, and military science. This tradition went back to Buddha’s time. Jotipala, the son of a Brahmin priest in the court of the King of Benares returned after graduating in archery and military science and was appointed the commander-in-chief. Jivika, Bimbisara’s physician who cured Buddha, learned medicine in Takshashila. Prasenajit, the king of Kosala, who too was associated with Buddha was educated in Takshashila.
Chanakya wanted to convert this cultural unity into political unity against the invader. For him, kingdoms of Ambhi and Porus, had to unite against the foreigner. He condemned foreign rule as exploitation; for the foreigner the conquered country was not his own, but a place to tax and extract wealth. He also realized that the reason Alexander was able to advance was because there was no united front: there was no leadership or pooling of resources. Alexander was able to exploit this division and was stopped only by a mutiny in his camp.
One of the first activities of Chanakya and his protegé Chandragupta was to organize resistance against the Greeks satrapies. We know this because of the writings of Justin, who was describing the return of Seleucus Nicator, an officer of Alexander  to India to expand the Greek kingdom.

Justin identified the leader of the rebellion as  Sandrocottus or Chandragupta Maurya.
There were six satrapies: three on the West of Indus and three on the East. Following Alexander’s departure, the satrapies he established started collapsing. At the same time, under the leadership of Chandragupta, a war was declared. The satraps Philip and Nicanor were assassinated and by 323 BCE, India was free of Greeks.
References:

1. Abraham Eraly, Gem in the Lotus: The Seeding of Indian Civilisation, 2005.
2. A. Dani, Historic City of Taxila (Bernan Press(PA), 1986).
3. Radhakumud Mookerji, Chandragupta Maurya and his times, 3rd ed. (Motilal Banarsidass, 1960).
4. John Keay, India: A History (Grove Press, 2001).