What was the Doctrine of Lapse? Why did Rani Laxmi Bai rise against the British?

Under the system of the Doctrine of Lapse, if an Indian ruler died without a natural heir, the adopted child would not have any right to inherit the throne. After the death of the ruler, his kingdom used to be annexed to the British territories.

Rani Laxmi Bai had to rise against the British because the kingdom of Jhansi was annexed by the British according to the Doctrine of Lapse. After the death of the ruler of Jhansi, Gangadhar Rao, the claims of his adopted son were disregarded by British and then Laxmi Bai had to rise to settle scores with the British.