Is another player missing a strategic resource you have? Keep it, unless that player offers to trade you another resource you need or something similarly generous. Are you neighbors with Scythia or Mongolia? Don't trade them any Horses. Slow your opponents down as much as possible: Remember what your opponents' strengths are, and do your best to deny them what they need.

Be wary, though: the closer you get to victory, the harder it will be to find players willing to ally with you. It means no declarations of war, no spying, and maybe even Alliance bonuses for 30 turns.

Team up if you can: When everyone can declare war on you, launch nuclear weapons at your cities, or conspire against you, having allies is more valuable than ever.

The Loyalty mechanics in the expansions make this doubly important, since cities close to an opponent's territory will lose Loyalty and eventually flip. The one possible exception to this rule is an island city, which generally doesn't anger opponents as much and will provide you with a good base for aircraft, building or repairing ships, and staging invasions late in the game (especially if it has an Aerodrome with an Airport to allow airlifting).

Don't overextend yourself: Expanding to a continent another player controls will most likely cause them to go to war with you and take your cities, and it will be nearly impossible to transport your army there quickly enough to protect them.

Which one(s) you focus most on improving will, of course, depend on your civilization, leader, and overall strategy. You never want to have the lowest Science output your development will be slowed, you'll be easy prey for warmongers, and even if you survive, you'll eventually lose diplomatic influence (since other players will have less trouble completing quests for city-states and you won't have a strong enough military to protect your interests). You'll need Production to quickly construct buildings and wonders and train units.

Improve your cities' yields: You'll need Food to grow your Population, work more hexes, and build more Districts in your cities.

Always keep some on hand for emergencies.

Watch your treasury: Gold can be used to purchase units or levy them from city-states, which will give you a much stronger defense and more options in wartime.

It's also advisable to ask for friendship with players of whom you're suspicious, which will prevent them from declaring or joining wars against you. You should also make sure you have a large enough military to defend yourself on all fronts - if you use your entire army and navy to attack your neighbor, another opponent might launch an attack on you while your territory is undefended. This ends the deal and leaves you unable to purchase units, while giving your opponent lots of Gold to spend. A cunning player may, for example, propose that you give them all the Gold in your treasury in exchange for most of their strategic and luxury resources, only to declare war on you the following turn. Warmongers are prevalent online, and are way smarter than AI opponents.

Never trust anyone: Don't expect any pity, especially from strangers.

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