India will NOT have credible deterence vis-a-vis China unless India posseses Megaton nukes
Because China does? The Chinese need for megaton nukes just shows the poor accuracy of their missiles. The UK uses a 100 kT (variant of the W76) warhead and their arsenal is very credible.
The radius of blast damage varies with the cube root of the warhead yield.
A 1MT weapon gives a 70% increase in blast and fire effects over a 200 kT weapon even though it is 500% larger in yield. You get a blast radius increase from 4.3 miles to 7.3 miles and you have to deliver the much larger and heavier 1 MT device.
While a 1 MT bomb would destroy buildings 5-7 miles away, a 27 MT bomb would destroy buildings 15-21 miles away. 27 times the yield to get 3 times the blast radius.
Compare 27 MT to 200kT.
135 times the yield but only 5-6 times the blast radius. And you need a monster ICBM to deliver that weapon. Largest weapon that SU fielded was 20MT for the SS-18 mod 1. This was designed to hit hardened command bunkers but its poor accuracy demanded a huge warhead. The Russian Topol-M uses a single 550 kT warhead for the same purpose.
A number of smaller bombs are far more efficient at causing destruction (yield to throw-weight) than a single large weapon.
That is why the US, Russia, UK etc all use kiloton yield warheads. The US W87 (used on MX and Minuteman 3 missiles) has a 300 kT yield.
Now, this only works if you have accurate missiles.. if your CEP is crap, then you cannot deliver the warheads in a dispersal pattern to ensure destruction. That is why the megaton weapons were used in the past. They paid the price in delivery of a large, heavy weapon in order to ensure they could destroy the target.
Making small weapons is much harder.. the Chinese had to steal the W88 warhead (300-475kT used on the Trident missile) design from the US. This uses a non-spherical (oblate) primary and a spherical secondary in a peanut shaped hohlraum.
French President Mitterand said that France did not have the capability to make small bombs like the US. Only after CTBT did they get design data.
We know from the traitor Vanunu's photos that the Israeli thermonuke is not a staged weapon like the Indian Shakti-1 design. It appears to be a much simpler sloika. You only get 50-60kT but that is enough to deter the arabs.
So 6 300kT warheads dropping on Chongqing or Shanghai will deter the lizard. If they have an ABM with 50% kill rate, then add three more -- 9 warheads to level the place.
Put those 9 warheads together with a decoy and chaff dispenser on a single heavy ICBM, or have three warheads each on three SLBMs dedicated for a Chinese city and they will think twice about territorial aggression.
We know from WOP book that Indian primaries in 1998 were spherical... so work needs to be done if MIRV capability is to be achieved.
Regarding the number of warheads that are enough to count us in the club, the genie is out of the bottle and the situation even 10-20 years from now is a total unknown.
If the security situation 10-20 years down the road demands that India must have 5000 bombs then India will build 5000 bombs. No treaty or US law will prevent this.
Look at Japan.. under PM Abe they are doing studies on cost and time frame for prototype nuke development. When they need to, they will say to hell with NPT and withdraw.
And you need superpower sized economy and military-industrial complex to have an arsenal of thousands of weapons and their delivery systems. One cannot eat grass and get this. China could not afford a large arsenal. They (and the French or the British) never sought to match the superpower arsenals.
And when you have such economy and MI complex, no treaty or domestic law of other states will stop you. A superpower has such freedom of action. Think about the world reaction if Abe announces a Japanese nuke tomorrow morning. Think there will be sanctions on Japan? Japan can withdraw from the CTBT and the NPT and the world has no choice but to accommodate them.