There is a saying: "United we stand, divided we fall". A single hand has a limited capacity, but more hands working together will have much more strength to apply on a particular thing at the same time. So we should be united in all our missions of life.
A single person's family is somewhat weak in the sense that there is hardly any other person to stand by his side in his calamity. But in a joint family, there are other members of the family who can come forward to one another's rescue in the days of calamity.
In case of a country also, the same example is applicable very appropriately. When the native land is attacked by any outside enemy, it is not a single or a few people's job to fight the enemy. It is the collective force of all the countrymen to fight out the evil.
Thus unity is always proved to be a great source of strength. It is really like an invincible constructive or destructive performance.