Complex sentence: Shock troops are chosen because they are thought to have high enough morale to take heavy casualties during the brief time of an attack, or are considered willing to take large losses based on political, religious, or other special motivation, such as reclaiming their homes.
Split into simpler sentences: Shock troops are chosen because they are thought to have high enough morale to take high casualties during the brief time of an attack. Or the perception that they were willing to take large losses might be based on political or religious indoctrination or because they had some special motivation, such as reclaiming their homes.