Block reward halves from 25 BTC to 12.5 BTC at block 420,000 on July 9, 2016. Bitcoin's price was around $650. The second halving confirmed that Satoshi's programmatic monetary policy would execute reliably every 210,000 blocks. Like the first halving, skeptics predicted a miner death spiral that never materialized. Instead, the reduced supply issuance preceded a historic bull run to nearly $20,000 by December 2017, reinforcing the stock-to-flow thesis among Bitcoiners.
Bitcoin Wiki: Controlled supply