Are we back to the 1930s? In the early 1930s, many people did not realise that regional tensions would lead the whole world into war.
War warning on day missile hits US cargo ship
Vital decade: Grant Shapps says the world order has been "shaken to its core". Glossary
USA - The United States of America is a country in North America. It has a population of 329 million.
Yemen - The poorest country in the Gulf region is in the midst of a humanitarian crisis.
Houthis - An Islamist rebel group that controls a large part of Yemen. They are allies of Hamas in Gaza.
Cargo - Goods that are being transported.
Red Sea - A seawater inlet of the Indian Ocean. To its right is Saudi Arabia, to its left the coast of north Africa.
Hamas - A militant Palestinian organisation, which has controlled the Gaza Strip since 2007. It is listed as a terrorist organisation in the UK.
Gaza - The smaller of two Palestinian territories, home to two million people. It is located on the Mediterranean coast, bordered by Israel and Egypt.
Israel - A country in the Middle East, and the only Jewish nation in the modern period. It is seen by Jews, Christians and Muslims as the Holy Land.
Iraq - A country in the Middle East with a population of 43.5m.
Iran - A country in Western Asia, officially called the Islamic Republic of Iran. It has a population of 84 million.
Atom bombs - An atom bomb rapidly releases nuclear energy.
Expensive - Costs a lot of money.