What is the Islamic ruling regarding a neighbor who seized a person's property (gas pipes, energy system) from his house without permission during the war and displacement, then refused to return them, using them for a year and a half, and then claimed they were damaged under the rubble? And is he obligated to guarantee what he took?

It is not permissible to transgress against people's property and seize it without a just cause. A Muslim's property is not lawful except with his permission and the contentment of his soul, even if it is a small thing. This has been indicated by texts from the Qur'an and Sunnah, and the consensus of the Ummah. Transgression against property is forbidden by the Qur'an, Sunnah, and consensus. Abu Al-Walid Ibn Rushd explained that transgression against property is divided into seven categories, all of which are forbidden: brigandage (hirabah), usurpation (ghasb), embezzlement (ikhtilas), theft (sariqa), betrayal (khiyanah), humiliation (idhlal), and denial and appropriation (al-jahd wal-iqtita'). The usurper must return the exact usurped item if it exists; otherwise, he must return its equivalent or its value.