Israel got surprised because there was no tactical depth (minefields, obstacles etc) and it is hard to be vigilant on defence all the time. Israel foolishly (for political reasons) relied upon ISR and firepower without any proper defensive belt (see Russian in Ukraine).
Hamas managed to brutally murder 1100 civilians and 150 soldiers. Within less than 24 hours Israel recovered and killed 1500 Hamas terrorists inside Israel with losses of less than 100 troops.
Hamas squandered their tactical surprise and successful penetration of Israeli defences by murdering babies, brutalising grandmas and generally behaving in the most despicable evil manner, worse than ISIS.
It was a terrorist "success" but a military failure.
Like Pearl Harbour it has united Israelis like never before and created a unified society dedicated to total war until its enemies are completely destroyed.
Lets look at the numbers.
Israel: Just called up 360k reserves (more than Russia did a year ago). This is on top of 180k standing army and 250k conscripts doing their compulsory service.
Total: 800,000 well trained troops with excellent weapons, many with combat experience and very high motivation in defence of homeland.
Reserve call up was 150% oversubscribed. I personally know elite forces people turned away because Israel had too many.
For the last 20 years or more 100,000 people a year did military service with many serving in the reserves. Israel has by far the youngest population of any 1st or 2nd world country. It could call up more than 2 million additional trained soldiers if necessary.
HAMAS is a terrorist group with no tanks, armoured vehicles, artillery. No heavy weapons at all. No air force. No navy. Its rockets are only good against fixed, civilian targets, not mobile military ones. Perhaps it could field 30,000 fighters, maybe 40,000 at the outside.
If Israel deploys 120k - 160k troops to Gaza there will be a 4:1 numerical superiority and a complete firepower overmatch in every department. Note that 3:1 is considered the needed attacking force advantage for equal quality forces.
It is true that Gaza is a fortress with lots of underground fortifications etc.
But so was Mariapol and Bukhmut, which had far better Soviet era underground cities and 8 years of NATO's billions fortifications.
Russia conquered both with only 20-30k troops in a few months for Mariopol (professional Russian army) and 8 months using Wagner.
Israeli troops are at least as good as the Russians facing a much weaker enemy with no support.
Conclusion: HAMAS will be destroyed.
