Complex sentence: The Tamworth Manifesto was a political manifesto issued by Sir Robert Peel in 1834 in Tamworth, which is widely credited by historians, as having laid down the principles, upon which the modern British Conservative Party is based.
Split into simpler sentences: The Tamworth Manifesto was a political manifesto issued by Sir Robert Peel in 1834 in Tamworth. Which is widely credited by historians, as having laid down the principles upon which the modern British Conservative Party is based.