(We are at war with Britain who is allied with Turkey, so I am assuming with are at war with Turkey)
1. A force of 80,000 troops land east of Istanbul 2. 120,000 troops march through European Turkey to Istanbul 3. The landing force moves west to cut off Istanbul 4. This move cuts off communications making the rest of Turkey in the dark about what's happening 5. with no food coming in, constant Artillery barrages, and the occasional assult, Istanbul will soon fall.