History has shown repeatedly that it's always easier to win a war with the strong support of allies. Allies are a group of nations, with common goals, joining to defeat their opposition. By pooling resources, allies have more of the necessary items, including machinery and labor, to win a war. It also helps create a larger network of bases for operations. However, there are downsides to having allies as well, including divergent opinions and possible financial ramifications.
Advantage: Pooled Resources
In a post-industrial world, many countries specialize in specific sectors of large-scale manufacturing rather than having a broad range of smaller operations. Allies can pool these specific resources together. For example, if country Y specializes in small arms manufacturing, it would do well to ally with country X, which has an established fighter-jet manufacturing base. This way, both countries are fully equipped, using each other's strengths. Additionally, pooled resources allow allies to increase their sheer labor. Joined together, the troops of countries Y and X create a larger force fighting for a common goal. Consider World War II; both the allies -- including the United States, United Kingdom, Russia, Poland, France and others -- and the Axis powers -- including Germany, Italy, Japan and others -- formed alliances. Each group significantly increased its labor and resources, with the common goal of fighting the opposition. Although the fighting took place in different parts of the world, the common goal remained to defeat the shared enemy.
Advantage: More Locations and Bases of Operation
Having allies means automatically having friendly ground from which to base operations. In both World War I and World War II, for instance, the United States joined in fighting several thousand miles from its domestic military bases. It could not have been as effective if not for the network of allied countries that allowed the U.S. to build bases and conduct military operations from their land. Post-war, many allied countries continue to allow each other to maintain military bases -- meaning there is an operational base that can be used in case of hostilities -- or to simply keep a military presence in the area. For example, the United States maintains extensive bases in South Korea following the Korean War, and soldiers continue to deploy there despite the lack of active war.
Disadvantage: Bailing Out Allies
When countries form an alliance, it is implied they will support each other, and that often includes post-conflict support. For instance, much of the destruction in World War II took place in France, and the fighting devastated the country. Allied powers, because of their alliance, had an obligation to help France recover -- at great financial cost. Helping a struggling ally often remains a responsibility even during times of no conflict. For example, the United States provides much of the finances and labor behind the North Atlantic Treaty Organization, an alliance between 28 North American and European countries that work cooperatively to keep peace in the North Atlantic region. In a June 2011 speech, U.S. Defense Secretary Robert Gates decried the alliance as one of increasing imbalance because other NATO members didn't contribute enough to operations, he said. Not only does the obligation to support struggling alliance members create financial hardships, but it also can create tension -- as Gates' speech demonstrated.
Disadvantage: Disagreement on Action
Although allies initially join to fight against a common enemy, sometimes tactics or goals change. For example, Israel and the United States are strong allies and American leaders have reaffirmed their solidarity with Israel repeatedly. The countries both need each other, but that doesn't mean leaders always agree. For example, in May 2011, U.S. President Barack Obama pressured Israel to return to 1967-era borders between Israel and Palestine as a way of brokering peace in the region. Israeli Prime Minister Benjamin Netanyahu flatly rejected the notion, putting the two allies at odds over an issue that is likely to reemerge repeatedly amid violence in the region. When fundamental disagreements between allies emerge, it can make the relationship uncomfortable or, in the worst-case scenario, end the alliance.