Once the conversations of WhatsApp in iOS have been eliminated, the most common thing we do is to restore them from the backup saved in iCloud. Note that you must create the backup before you delete the message history in WhatsApp, you can do it by accessing:
WhatsApp> Settings> Chat settings> Copy of chats.
Select "Make a copy now" or activate a schedule to automatically make copies with the "Automatic copy". The backup is stored in your iCloud account.
Then, remove the application and reinstall it on the iOS device.
Verify our phone number and select restore backup, in a few seconds you will see how they are all.
You can also recover old messages deleted from WhatsApp from backup that is done through iCloud or iTunes.