By land
If the said enemy is to attack via land forces, it has to take control of Malaysia before attacking Singapore. Part of Singapore's army (land forces) are on 24/7 stand-by, ready for immediate mobilisation. It is also document in books by military experts that Singapore has a rapid deployment force that can march into Malaysia within 40minutes. Singapore's forces is also said to be capable of securing land up to Johor's state line thus securing additional water catchments & farmland for food. Johor will also act then as a buffer for artillery attacks, sparing the mainland from massive damage.