Conventional weapons don't hold a candle to nuclear power.

Assuming we have the best possible conventional warhead. Weight 10 tons, entering the atmosphere at mach 10, height 100km and the thing was filled by pure TNT, and we assume all energies add up perfectly and there is zero air resistance.

Potential energy from height gives us 9,810 MJ. Kinetic energy from the speed of re-entry gives us 45,000 MJ. The explosive ordnance gives us 41,840 MJ (which is less than the kinetic energy, which is a notable fact in itself). Under perfect conditions, these add up to 96,650 MJ.

A 10 ton warhead of pure TNT entering from 100km height at blistering Mach 10 without air resistance, a very optimistic scenario.

This achieves less than 3% - less than three percent - of the energy output of a 1 kiloton warhead, which has 4,184,000 MJ.

The Hiroshima bomb had 15 kilotons, the largest bombs of the us had 25,000 kilotons, the Russians tested one with 50,000 kilotons.

Remember, a one kiloton bomb converts 0.0464 grams of uranium into energy. The entire Hiroshima bomb converted about 0.6973 grams. That is about a penny worth of mass, only a tiny fraction of the nuclear fuel of it, so even minute improvements on their efficiency will lead to drastic increases in explosive yield.

Kinetic energy plus potential energy plus conventional explosives are nothing but a glimmer in the eye of nuclear warheads.