GAZA CITY, Gaza Strip (AP) — Thousands of Israeli troops backed by columns of tanks and helicopter gunships launched a ground offensive in Gaza Saturday night, with officials saying they expected a lengthy fight in the densely populated territory after eight days of punishing airstrikes failed to halt militant rocket attacks on Israel.

The incursion set off fierce clashes with Palestinian militants and Gaza’s Hamas rulers vowed the coastal strip would be a “graveyard” for Israelis forces.

“This will not be easy and it will not be short,” Israeli Defense Minister Ehud Ba...