According to you, who was the cause of the Mahabharata War? Is it Shantanu or Bhishma?

 Karna at the last moment before the Kurukshetra war rejected Shri Krishna's proposal for his (Karna's) instating as the monarch sovereign of jambudwipa (the entire humankind existence at that time), from Suraalaya & Uttara Kuru (the northernmost areas of human existence during mahabharata, modern day countries of Kyrgyzstan, Tajikistan, Kazakhstan, and Uzbekistan, and western parts of Tibet), the vast land of Aryavrata in the middle (northern Indian subcontinent and neighbouring areas), upto Dakshinatya in the south (the Deccan Plateau and eventually Lankapuri), also the proposal of Shri Krishna to Karna for Karna to get married to Draupadi as her sixth and eldest husband, after Yudhishthira, Bhima, Arjuna, Nakula, and Sahadeva.

• Shri Krishna's proposal (to Karna) as Karna the absolute monarch of Jambudwipa

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The brothers, the five Pandavas, the son of Draupadi, and the invincible son of Subhadra, will all embrace thy feet. All the kings and princes, again, that have been assembled for the Pandava-cause, and all the Andhakas and Vrishnis, will also embrace thy feet. Let queens and princesses bring golden and silver and earthen jars (full of water) and delicious herbs and all kinds of seeds and gems, and creepers, for thy installation. During the sixth period, Draupadi also will come to thee (as a wife). Let that best of Brahmanas, Dhaumya, of restrained soul, pour libations of clarified butter on the (sacred) fire, and let those Brahmanas regarding all the four Vedas as authoritative (and who are acting as priests unto the Pandavas), perform the ceremony of thy installation. Let the family priest of the Pandavas who is devoted to Vedic rites, and those bulls among men-those brothers, the five sons of Pandu,--and the five sons of Draupadi, and the Panchalas, and the Chedis, and myself also, install thee as the lord of the whole earth. Let Dharma's son Yudhishthira, of righteous soul and rigid vows, be thy heir presumptive, ruling the kingdom under thee. Holding the white chamara in his hand (for fanning thee), let Yudhishthira, the son of Kunti, ride on the same car behind thee. After thy installation is over, let that other son of Kunti, the mighty Bhimasena, hold the white umbrella over thy head. Indeed, Arjuna then will drive thy car furnished with a hundred tinkling bells, its sides covered with tiger-skins, and with white steeds harnessed to it. Then Nakula and Sahadeva, and the five sons of Draupadi, and the Panchalas with that mighty car-warrior Sikhandin, will all proceed behind thee. I myself, with all the Andhakas and the Vrishnis, will walk behind thee. Indeed, all the Dasarhas and the Dasarnas, will, O king, be numbered with thy relatives. Enjoy the sovereignty of the earth, O thou of mighty arms, with thy brothers the Pandavas, with yapas and homas and auspicious rites of diverse kinds performed in thy honour.

• Shri Krishna offered the following to Karna
• The sovereign monarch of Jambudwipa (the entire human existence during mahabharata) without a second opponent
• The sibling brothers of Karna, namely, Yudhishthira, Bhima, Arjuna, along with Nakula and Sahadeva, shall obey his sovereignty, and shall serve as the princes of Jambudwipa under his command, and shall performs conquests (if necessary any) solely for his sovereignty
• Draupadi as the wife of Karna along with Karna's other wives, so that Karna could father a son with Draupadi like his brothers, the Pandavas
• The coronation and rule of Karna as the supreme sovereign of Jambudwipa, with the best of brahmanas and knowers of dharma, along with the best of queens and princesses of various kingdoms of aryavrata under Karna's rule, performing a grand ceremony of Karna's coronation, and the five Pandavas (Yudhishthira, Bhima, Arjuna, Nakula, Sahadeva), the brothers of Karna, along with Shri Krishna & the Yadu race, themselves, actively participating in the coronation ceremony of Karna as the emperor of aryavrata

However Karna rejected such a grand proposal of Shri Krishna (perhaps the grandest proposal ever made by any mahabharata character, that too Shri Krishna himself), to evade an all-out war and the ambition to see himself as the sovereign ruler of the entire earth at that time, solely due to his intimate bond with his inseparable friend Duryodhana, the Kuru prince, whom Karna wished to see as the emperor of Aryavrata, and whom (Duryodhana) Shri Krishna, the epitome of dharma, would have been the last person to visualise as the emperor of aryavrata, since Shri Krishna, the knower of everything, knew that Duryodhana was the incarnation of Kali Purush (the evil Kali Yuga, the era of morale deterioration and adharma), and the unjust rule of Duryodhana would have led to the establishment of adharma, greed, the six sins, and all kinds of unrighteousness possible.

Karna accepting the proposal of Shri Krishna, as the sovereign of bharatvarsha , the husband of Draupadi, just at the advent of Kurukshetra could have easily prevented the all-devastating war itself. But unfortunately was not.

Image courtesy : Perplexity AI

Why didn't Bhishma kill Kansa if he was powerful in reality?

 Do you think Kamsa was some nobody that Bhishma could just invade and kill him ? Kamsa was extremely powerful and had defeated Indra and the Devas in battle.

1. Description of Kamsa’s strength from Harivamsa -

O lord of the world, having slain the army of Jarasandha and vanquished the other enemies in battle why hast you met with death at the hands of an ordinary man? Alas! Fighting with Indra in a combat of arrows you were not defeated by the immortals. How hast you been then slain by a mortal? Having agitated, with a shower of arrows, the ocean that is incapable of being moved you didst conquer Varuna the holder of the noose, and take away all his riches. When Vasava did not pour profuse showers, you didst, for the citizens, bore through the clouds with your arrows and bring down rain by force. By your prowess all kings were humiliated and used to send you the precious jewels and clothes.

• Chapter 31, Vishnu parva, Harivamsa.

2. Vishnu Purana says that Kamsa defeated the Devas including Indra -

I scorn the Ādityas, accompanied by the timid Vasus. I scorn the Agnis and all the other deities defeated by my mighty arms. Did I not see Indra, king of gods, retreating from the battlefield with arrows in his back? He couldn’t take them on the chest. When Indra stopped the rains from falling on my kingdom, did not the clouds, split asunder by my missiles, release their showers, just as I desired?

• Chapter 4, Book 5, Vishnu Purana.

3. Srimad Bhagavatam also says that Kamsa defeated the Devas -

What can the gods do with all their endeavours. They are cowards on the battlefield. They live in perpetual terror of the twanging of your bow-string. Being beaten on all sides with volleys of arrows discharged by you, they, being anxious to save their lives, deserted the battlefield and took to their heels. Some gods in their wretched plight laid down their arms and stood with folded palms, while others with their loincloth untucked and hair on the head untied and dishevelled announced, “We are afraid.”

• Chapter 4, 10th Skandha, Srimad Bhagavatam.

4. Garga Samhita has an elaborate description of Kamsa’s battle with Indra -

With a hard punch Kaṃsa struck the great elephant and with a second punch he struck Indra in that battleground. With that punch Indra fell far away and the elephant was wounded and fell, its knees touching the ground. Again standing, the king of elephants attacked the demon-king Kaṃsa with its tusks, picked him up with its trunk, and threw him eight-hundred thousand miles. His body hard as a thunderbolt, when he fell he was only a little upset in his heart. Angry, and his lips trembling, he returned to the battleground. Kaṃsa grabbed the elephant-king Airāvata, threw him onto the battleground, strangled its trunk, and broke its tusks into pieces. Knocking over many great heroes, the elephant Airāvata fled the battleground and went to the capitol of the demigods. Taking the bow of Lord Viṣṇu and stringing it, with a flood of arrows accompanied by the twanging sound of the bow, the demon-king Kaṃsa made the demigods flee.

• Chapter 7, Goloka Khanda, Garga Samhita.

So Kamsa was an extremely powerful ruler. He was also aided by powerful Asuras. Bhishma probably don't want to risk a battle with Such a powerful king. Only Lord Krishna could have killed Kamsa easily.