In the early 1500s the Portuguese invaded, building one of their typical sea-facing forts on Bahrain’s northern shore (Qala’at al-Bahrain) – the coping stone on seven layers of ancient history. Their rule was short-lived, however, and by 1602 the Portuguese were ousted by the Persians.
It was in the mid-18th century that the Al-Khalifa, the family that now rules Bahrain, first arrived in the area. They initially settled at Al-Zubara, on the northwestern edge of the Qatar peninsula, and became involved in the region’s lucrative pearling trade. They drove the Persians out of Bahrain in about 1782, and were themselves routed by an Omani invasion, but returned in 1820 never to leave again.
During the 19th century, piracy was rife in the Gulf and, although it never gained a foothold in Bahrain as such, the island gained something of a reputation as an entrepôt, where pirates sold their captured goods and bought supplies for the next raid. The British, anxious to secure their trade routes with India, brought the Al-Khalifa family, who were professedly opposed to piracy, into the ‘Trucial system’ (the system of protection against piracy that operated throughout the old Trucial States; that is, the Gulf states which signed a ‘truce’ or treaty with Britain against piracy and which largely make up today’s UAE). In hindsight, this could almost be dubbed ‘invasion by stealth’, as by 1882 Bahrain could not make any international agreements or host any foreign agent without British consent. On the other hand, as a British protectorate, the autonomy of the Al-Khalifa family was secure and threats from the Ottomans thwarted. To this day, a special relationship can be felt between the Bahrainis and the sizable expatriate British community, even if only in the landscaping of public parks and the building of roundabouts. Bahrain regained full independence in 1971.
http://www.lonelyplanet.com/bahrain/historyNot WW1 related at all - the ruling family first installed themselves over 100 years earlier, and later used British help to stay, giving up effective independence in the process.
The 'ousting' of the Portuguese shows that the Persians were quite used to aggressive warfare; or the
conquest of Baghdad by Shah 'Abbas. The Persians were just like any other empire - they fought, and took control of places when they were able to.