Pierre, Natasha, Andrei, Nikolai, and Marya are all connected to inappropriate partners, mostly because of their youth and general inexperience, and also because of their individual personality quirks.
Napoleon's 1812 invasion of Russia disrupts everyone's lives, dramatically transforming each character's personality because of demanding new circumstances, and killing off Andrei.
After the war, those who have survived re-form into lasting and meaningful family bonds, rebuilding the domestic lives that have suffered upheaval.