Complex sentence: Corunna was not chosen, as it was seen as being too close to the American border--in the 1820s-1830s, the prospect of an Irish Fenian raid from the United States was considered a serious threat to the British colonies.
Split into simpler sentences: Corunna was not chosen, as it was seen as being too close to the American border. In the 1820s-1830s, the prospect of an Irish Fenian invasion from the U.S. was considered a serious threat to the British colonies.