(Have they not traveled), `these people who disbelieve in your Message, O Muhammad,'
(in the land and seen what was the end of those who were before them) means, the nations of the past who disbelieved in their Prophets (peace be upon them), for which the punishment came upon them even though they were stronger than Quraysh.
(and in the traces in the land.) means, they left behind traces in the earth, such as structures, buildings and dwellings which these people i.e., the Quraysh cannot match. This is like the Ayat:
(And indeed We had firmly established them with that wherewith We have not established you!) (46:26)
(and they tilled the earth and populated it in greater numbers than these have done) (30:9). Yet despite this great strength, Allah punished them for their sin, which was their disbelief in their Messengers.
(And none had they to protect them from Allah.) means, they had no one who could protect them or ward off the punishment from them. Then Allah mentions the reason why He punished them, and the sins which they committed. Allah says:
(That was because there came to them their Messengers with clear evidences) meaning with clear proof and definitive evidence.
(but they disbelieved.) means, despite all these signs, they disbelieved and rejected the Message.
(So Allah seized them.) means, He destroyed them utterly, and a similar fate awaits the disbelievers.
(Verily, He is All-Strong, Severe in punishment) means, He is possessed of great strength and might.
(Severe in punishment, ) means, His punishment is severe and agonizing; we seek refuge with Allah, may He be blessed and exalted, from that.