When the civil war in Syria threatened his village more than a decade ago, a farmer and his family fled to neighboring Lebanon.

The farmer, Ali Kheir Khallu, 37, found work there growing oranges and bananas. Life was hard, he said, but at least he felt safe.

That feeling vanished last month as Israel ramped up its war with Hezbollah, the powerful Lebanese militia, heavily bombing sites that it said belonged to the group. When the bombs fell near Mr. Kheir Khallu’s house, he packed up his family, left behind the new lives they had built in Lebanon and fled back to Syria, where they are now struggling to start over, yet again.

“You want to make up for all that you have lost,” he said. “But you are still in shock.”

As the war in Lebanon expands, more than 1.2 million people — one-fifth of the population — have been displaced from their homes, the government says.

While most have sought safety in other parts of Lebanon, more than 470,000 people, mostly Syrians, have crossed into Syria in the last six weeks, aid groups say.

Since Syrian rebels tried to topple the government in 2011, President Bashar al-Assad has fought to stay in power, with his forces bombing and besieging opposition communities and repeatedly using chemical weapons. The war drew in Russia, the United States, the jihadists of Islamic State and other forces, displacing about 12 million residents, or more than half the country’s population.